Add plus upstream metrics

[aphralG]

Proposed changes

• Added plus stream upstream response time metrics

  • plus.stream.upstream.peers.response.time.count
  • plus.stream.upstream.peers.response.time.max
  • plus.stream.upstream.peers.response.time.median
  • plus.stream.upstream.peers.response.time.pctl95

• Added plus http upstream response time metrics

  • plus.http.upstream.peers.response.time.count
  • plus.http.upstream.peers.response.time.max
  • plus.http.upstream.peers.response.time.median
  • plus.http.upstream.peers.response.time.pctl95

• Added plus upstream connect time metrics

  • plus.stream.upstream.connect_time.count
  • plus.stream.upstream.connect_time.max
  • plus.stream.upstream.connect_time.median
  • plus.stream.upstream.connect_time.pctl95

• Added plus upstream header time metrics

  • plus.http.upstream.peers.header_time.count
  • plus.http.upstream.peers.header_time.max
  • plus.http.upstream.peers.header_time.median
  • plus.http.upstream.peers.header_time.pctl95

• Added nginx.http.request.current to the nginx oss metrics

Closes

closes #228

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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