Add event processing histogram metric

[kate-osborn]

Proposed changes

Problem: NGF does not measure how long it takes to process an event batch.

Solution: Add a new histogram metric event_batch_processing_milliseconds, that measures the time it takes to process an event batch. Also adds a debug log statement with the same information so we can debug spikes in processing time.

Testing: Tested that the new metric is exported and that no errors occur when metric collection is disabled.

Please focus on (optional): I added a new Prometheus collector that handles this new metric instead of adding this metric to the existing nginx runtime collector. I think it makes sense to keep this metric separate from the nginx runtime metrics in case we end up splitting the control and data planes.

Closes #1131

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork