stats: Add universal stats tag from CLI (#18668)

Fixes #1975
Adds a CLI parameter that can be repeated that acts as universal tags for all stats

Additional Description:
Risk Level: Low, additional parameter
Testing: Unit + integration + manual
Docs Changes: Provided
Release Notes: Pending
Platform Specific Features: N/A

Signed-off-by: Sotiris Nanopoulos <sonanopo@microsoft.com>

Mirrored from https://github.com/envoyproxy/envoy @ c75c1410c8682cb44c9136ce4ad01e6a58e16e8e

envoy/admin/v3/server_info.proto

@@ -58,7 +58,7 @@ message ServerInfo {
   config.core.v3.Node node = 7;
 }
 
-// [#next-free-field: 38]
+// [#next-free-field: 39]
 message CommandLineOptions {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.admin.v2alpha.CommandLineOptions";
@@ -189,4 +189,7 @@ message CommandLineOptions {
 
   // See :option:`--enable-core-dump` for details.
   bool enable_core_dump = 37;
+
+  // See :option:`--stats-tag` for details.
+  repeated string stats_tag = 38;
 }