Memoize Metric::Capture.capture_cols
Memoize Metric::Capture.capture_cols

capture_cols will always be the same regardless of when it is called,
unless a database update happens to the metrics table, in which the
process would be restarted after the migration has happened, and it will
get reloaded then.

This method is hit frequently on the metrics collectors, so this
prevents a bunch of objects from being allocated multiple times when
doing perf_capture_realtime:

On a sample benchmark for perf_capture_realtime:

Before:  20172 objects allocated from this method
After:   None (after being memoize)

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