docs: mention multi-tenancy in docs (#4357)

The update should make understanding of multi-tenancy more clear
for influxdb users.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

README.md

@@ -528,8 +528,10 @@ and stream plain InfluxDB line protocol data to the configured TCP and/or UDP ad
 
 VictoriaMetrics performs the following transformations to the ingested InfluxDB data:
 
-* [db query arg](https://docs.influxdata.com/influxdb/v1.7/tools/api/#write-http-endpoint) is mapped into `db` label value
-  unless `db` tag exists in the InfluxDB line. The `db` label name can be overridden via `-influxDBLabel` command-line flag.
+* [db query arg](https://docs.influxdata.com/influxdb/v1.7/tools/api/#write-http-endpoint) is mapped into `db` 
+  [label](https://docs.victoriametrics.com/keyConcepts.html#labels) value unless `db` tag exists in the InfluxDB line. 
+  The `db` label name can be overridden via `-influxDBLabel` command-line flag. If more strict data isolation is required,
+  read more about multi-tenancy [here](https://docs.victoriametrics.com/keyConcepts.html#multi-tenancy).
 * Field names are mapped to time series names prefixed with `{measurement}{separator}` value, where `{separator}` equals to `_` by default. It can be changed with `-influxMeasurementFieldSeparator` command-line flag. See also `-influxSkipSingleField` command-line flag. If `{measurement}` is empty or if `-influxSkipMeasurement` command-line flag is set, then time series names correspond to field names.
 * Field values are mapped to time series values.
 * Tags are mapped to Prometheus labels as-is.

docs/README.md

@@ -531,8 +531,10 @@ and stream plain InfluxDB line protocol data to the configured TCP and/or UDP ad
 
 VictoriaMetrics performs the following transformations to the ingested InfluxDB data:
 
-* [db query arg](https://docs.influxdata.com/influxdb/v1.7/tools/api/#write-http-endpoint) is mapped into `db` label value
-  unless `db` tag exists in the InfluxDB line. The `db` label name can be overridden via `-influxDBLabel` command-line flag.
+* [db query arg](https://docs.influxdata.com/influxdb/v1.7/tools/api/#write-http-endpoint) is mapped into `db` 
+  [label](https://docs.victoriametrics.com/keyConcepts.html#labels) value unless `db` tag exists in the InfluxDB line. 
+  The `db` label name can be overridden via `-influxDBLabel` command-line flag. If more strict data isolation is required,
+  read more about multi-tenancy [here](https://docs.victoriametrics.com/keyConcepts.html#multi-tenancy).
 * Field names are mapped to time series names prefixed with `{measurement}{separator}` value, where `{separator}` equals to `_` by default. It can be changed with `-influxMeasurementFieldSeparator` command-line flag. See also `-influxSkipSingleField` command-line flag. If `{measurement}` is empty or if `-influxSkipMeasurement` command-line flag is set, then time series names correspond to field names.
 * Field values are mapped to time series values.
 * Tags are mapped to Prometheus labels as-is.

docs/Single-server-VictoriaMetrics.md

@@ -539,8 +539,10 @@ and stream plain InfluxDB line protocol data to the configured TCP and/or UDP ad
 
 VictoriaMetrics performs the following transformations to the ingested InfluxDB data:
 
-* [db query arg](https://docs.influxdata.com/influxdb/v1.7/tools/api/#write-http-endpoint) is mapped into `db` label value
-  unless `db` tag exists in the InfluxDB line. The `db` label name can be overridden via `-influxDBLabel` command-line flag.
+* [db query arg](https://docs.influxdata.com/influxdb/v1.7/tools/api/#write-http-endpoint) is mapped into `db` 
+  [label](https://docs.victoriametrics.com/keyConcepts.html#labels) value unless `db` tag exists in the InfluxDB line. 
+  The `db` label name can be overridden via `-influxDBLabel` command-line flag. If more strict data isolation is required,
+  read more about multi-tenancy [here](https://docs.victoriametrics.com/keyConcepts.html#multi-tenancy).
 * Field names are mapped to time series names prefixed with `{measurement}{separator}` value, where `{separator}` equals to `_` by default. It can be changed with `-influxMeasurementFieldSeparator` command-line flag. See also `-influxSkipSingleField` command-line flag. If `{measurement}` is empty or if `-influxSkipMeasurement` command-line flag is set, then time series names correspond to field names.
 * Field values are mapped to time series values.
 * Tags are mapped to Prometheus labels as-is.

docs/guides/migrate-from-influx.md

@@ -45,10 +45,12 @@ with similarities and differences:
   or [fields](https://docs.influxdata.com/influxdb/v2.2/reference/key-concepts/data-elements/#field-key) in
   VictoriaMetrics, metric name contains it all. If measurement contains more than 1 field, then for VictoriaMetrics
   it will be multiple metrics;
-* there are no [buckets](https://docs.influxdata.com/influxdb/v2.2/reference/key-concepts/data-elements/#bucket)
-  or [organizations](https://docs.influxdata.com/influxdb/v2.2/reference/key-concepts/data-elements/#organization), all
+* there are no [databases](https://docs.influxdata.com/influxdb/v1.8/concepts/glossary/#database), 
+  [buckets](https://docs.influxdata.com/influxdb/v2.2/reference/key-concepts/data-elements/#bucket)
+  or [organizations](https://docs.influxdata.com/influxdb/v2.2/reference/key-concepts/data-elements/#organization). All
   data in VictoriaMetrics is stored in a global namespace or within
-  a [tenant](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#multitenancy).
+  a [tenant](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#multitenancy). 
+  See more about multi-tenancy [here](https://docs.victoriametrics.com/keyConcepts.html#multi-tenancy). 
 
 Let's consider the
 following [sample data](https://docs.influxdata.com/influxdb/v2.2/reference/key-concepts/data-elements/#sample-data)

docs/keyConcepts.md

@@ -34,6 +34,8 @@ You can be more specific here by saying `requests_success_total` (for only succe
 or `request_errors_total` (for requests which failed). Choosing a metric name is very important and supposed to clarify
 what is actually measured to every person who reads it, just like **variable names** in programming.
 
+#### Labels
+
 Every metric can contain additional meta-information in the form of label-value pairs:
 
 ```
@@ -53,6 +55,12 @@ requests_total{path="/", code="200"}
 {__name__="requests_total", path="/", code="200"} 
 ```
 
+Labels can be automatically attached to the [time series](#time-series) 
+written via [vmagent](https://docs.victoriametrics.com/vmagent.html#adding-labels-to-metrics) 
+or [Prometheus](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#prometheus-setup).
+VictoriaMetrics supports enforcing of label filters for [query API](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#prometheus-querying-api-enhancements)
+to emulate data isolation. However, the real data isolation can be achieved via [multi-tenancy](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#multitenancy).
+
 #### Time series
 
 A combination of a metric name and its labels defines a `time series`. For example,
@@ -344,6 +352,18 @@ It is very important to keep under control the number of unique label values, si
 leads to a new [time series](#time-series). Try to avoid using volatile label values such as session ID or query ID in order to
 avoid excessive resource usage and database slowdown.
 
+### Multi-tenancy
+
+[Cluster version](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html) of VictoriaMetrics 
+supports [multi-tenancy](https://docs.victoriametrics.com/Cluster-VictoriaMetrics.html#multitenancy)
+for data isolation.
+
+Multi-tenancy can be emulated for [single-server](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html) 
+version of VictoriaMetrics by adding [labels](#labels) on [write path](#write-data)
+and enforcing [labels filtering](https://docs.victoriametrics.com/Single-server-VictoriaMetrics.html#prometheus-querying-api-enhancements) 
+on [read path](#query-data).
+
+
 ## Write data
 
 VictoriaMetrics supports both models used in modern monitoring applications: [push](#push-model) and [pull](#pull-model).