### Describe Your Changes
Added an ability to query data across multiple tenants. See:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1434
Currently, the following endpoints work with multi-tenancy:
- /prometheus/api/v1/query
- /prometheus/api/v1/query_range
- /prometheus/api/v1/series
- /prometheus/api/v1/labels
- /prometheus/api/v1/label/<label_name>/values
- /prometheus/api/v1/status/active_queries
- /prometheus/api/v1/status/top_queries
- /prometheus/api/v1/status/tsdb
- /prometheus/api/v1/export
- /prometheus/api/v1/export/csv
- /vmui
A note regarding VMUI: endpoints such as `active_queries` and
`top_queries` have been updated to indicate whether query was a
single-tenant or multi-tenant, but UI needs to be updated to display
this info.
cc: @Loori-R
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Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>
Signed-off-by: f41gh7 <nik@victoriametrics.com>
Co-authored-by: f41gh7 <nik@victoriametrics.com>
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