Track SPP push blocked secrets found metric

What does this MR do and why?

This adds a new event/metric for tracking how many times a push was blocked (secrets detected with no errors) by Secret Push Protection.

Additional Context

Secret Push Protection is an ultimate feature, however, there's a plan to enable the feature for all public projects regardless of the tier, as part of this, we're dark-launching the feature (i.e. enabling it for all public projects, without blocking pushes) to understand the load and monitor if that will cause any performance issues. This dark-launch is currently at 10%.

So while the feature's code still resides in EE, I was hoping to have the events tracked for all tiers.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/562705

MR acceptance checklist

I have evaluate this MR against the MR acceptance checklist.