Average memory metrics instead of summing

[Giners]

Proposed changes

This PR makes changes to how the following metrics are calculated:

• nginx.workers.mem.vms
• nginx.workers.mem.rss

Previously they were calculated by taking the sum of all the measurements over a polling period. Now they are to be averaged. This aligns with how the metrics are expected to be displayed in Amplify.

Checklist

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• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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