refactor(flags): Standardize tombstone counter labels across Python and Rust

[haacked]

Problem

In our Grafana dashboard, I noticed the tombstone panel had an entry like posthog//flag_dependency_missing. That's because the Tombstone metrics in Rust don't match the schema of the metrics in Python.

Changes

• Use consistent label names: namespace, operation, component
• Remove high-cardinality labels (team_id, flag_id) from metrics
• Log details separately to avoid cardinality issues
• Update TOMBSTONE_COUNTER description to document this approach

How did you test this code?

Unit Tests

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Changelog: (features only) Is this feature complete?