Redesign push-event metrics for lower cardinality and lower churn

[darthfork]

Parent issue: #34

Summary

Push event metrics currently increment per commit and include author-identifying labels, which can create excessive series churn and poor scalability for repositories with high commit volume.

Goals

• Revisit whether push metrics should be per-push, per-commit, or both.
• Avoid author-email based labels by default.
• Prefer aggregate metrics that stay useful without exploding cardinality.

Possible directions

• push_events_total{repository, branch_class}
• commits_pushed_total{repository, branch_class}
• optional author-level metrics behind config

Acceptance criteria

• Push metrics remain useful while avoiding unbounded author/email dimensions.