[GCP] Migrate gcp.metrics metricset to data stream

[endorama]

Some metricsets were not migrated in #490. gcp.metrics was one of those.

There is an external contribution that should be re-opened and worked upon #2314

[endorama]

After discussing how to proceed on this we decided we prefer a different approach than migrating this metricset to a data stream.
Recently the team working on package-spec added support for a new package type, called input.

The gcp.metrics metricset is the perfect candidate for an input package, as it is a metricset that require advanced configuration in order to be used by end user and this pose some additional challenges (mainly around the ingest pipeline backing this data stream).
We prefer to focus on making it an input package as it gives a better experience for end users and enables more advanced and complex use cases.

This issue will be closed in favour of one related to the input package implementation. At the moment input packages still require support in Fleet, so we will resume working on this for the next iteration (we plan to target 8.6.0).