lib/envflag: add -envflag.enable command-line flag for enabling reading flags from environment vars

By default flags are read only from command line. They can be read from environment vars if `-envflag.enable` is set.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/311

lib/envflag/envflag.go

@@ -5,6 +5,10 @@ import (
 	"os"
 )
 
+var enable = flag.Bool("envflag.enable", false, "Whether to enable reading flags from environment variables additionally to command line. "+
+	"Command line flag values have priority over values from envoronment vars. "+
+	"Flags are read only from command line if this flag isn't set")
+
 // Parse parses environment vars and command-line flags.
 //
 // Flags set via command-line override flags set via environment vars.
@@ -12,6 +16,9 @@ import (
 // This function must be called instead of flag.Parse() before using any flags in the program.
 func Parse() {
 	flag.Parse()
+	if !*enable {
+		return
+	}
 
 	// Remember explicitly set command-line flags.
 	flagsSet := make(map[string]bool)
@@ -25,6 +32,7 @@ func Parse() {
 			// The flag is explicitly set via command-line.
 			return
 		}
+		// Get flag value from environment var.
 		if v, ok := os.LookupEnv(f.Name); ok {
 			f.Value.Set(v)
 		}