Add upstream connection time and header time metrics

[Dean-Coakley]

Proposed changes

• Add the following new metrics:
  • nginx.upstream.connect.time
  • nginx.upstream.connect.time.count
  • nginx.upstream.connect.time.max
  • nginx.upstream.connect.time.median
  • nginx.upstream.connect.time.pctl95
  • nginx.upstream.header.time
  • nginx.upstream.header.time.count
  • nginx.upstream.header.time.max
  • nginx.upstream.header.time.median
  • nginx.upstream.header.time.pctl95
• Improve performance of request time metrics

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• 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
• I have updated any relevant documentation (README.md)

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[dhurley]

Looks good. You just need to run make deps