istio: only count master and release branches

[dgn]

There was a miscount previously where private branches on repositories could be used to produce misattributions. Limiting the branches that are taken into account to master and release-[0-9].[0-9]+ removes that problem.

Adding metrics/istioproject_developer_stats.sql which is a copy of metrics/shared/project_developer_stats.sql except for the following diff:

$ diff metrics/shared/project_developer_stats.sql metrics/istio/project_developer_stats.sql 
97a98,101
> left join
>   gha_payloads p
> on
>   p.event_id = e.id
99a104,108
>   and (
>     e.type != 'PushEvent'
>     or p.ref in ('refs/heads/master', 'refs/heads/main')
>     or p.ref ~ '^refs/heads/release-[0-9]+\.[0-9]+$'
>   )

which aims to filter out all PushEvents that are on unsupported branches (ie not on master or release branches).

Please make sure that you follow instructions from CONTRIBUTING

Specially:

• Check if all tests pass, see TESTING for deatils.
• Make sure you've added test coverage for new features/metrics.
• Make sure you have updated documentation.
• If you added a new metric, please make sure you have been following instructions about adding new metric.

[lukaszgryglicki]

cc @caniszczyk to approve this.