Remote Execution: add build target's tags to action's Request Metadata

[sluongng]

Description of the feature request:

We want to capture the "tags" that users added to each build target when remote execution is enabled. Effectively, add 1 field in remote_execution.proto RequestMetadata for repeated string target_tags would be desirable

--- a/build/bazel/remote/execution/v2/remote_execution.proto
+++ b/build/bazel/remote/execution/v2/remote_execution.proto
@@ -2118,4 +2118,7 @@ message RequestMetadata {
   // There is no expectation that this value will have any particular structure,
   // or equality across invocations, though some client tools may offer these guarantees.
   string configuration_id = 7;
+
+  // Multiple group identifiers for the target which produced this action.
+  repeated string target_tags = 8;
 }

Which category does this issue belong to?

No response

What underlying problem are you trying to solve with this feature?

This would help aid statistic collection, and action classifications on the server side for historical data aggregation. This lets our users tag their targets with generic tags without affecting cache key calculation.

Some example use cases:

• Attaching team ownership to each build target and action. Aiding on-call process when failure happens.
• Ability to classify, and associating costs of running actions to different generic categories (programming languages, services, teams, etc...)

Which operating system are you running Bazel on?

N/A

What is the output of bazel info release?

No response

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

[coeuvre]

It seems like a protocol change. Can you open a PR in https://github.com/bazelbuild/remote-apis to discuss there?