Prometheus metrics for senddata and git archive cache

This adds some new prometheus metrics:

• senddata hijackings by hijacker
• senddata response bytes by hijacker
• 'git archive' cache hits and misses

assigned to @nick.thomas

I forget - do any of our injectors rely on the http.Hijacker or http.Flusher interfaces? This will break them if so, won't it?

I will double-check but I don't think they do.

We could do some dynamic tomfoolery in NewCountingResponseWriter to pass on Hijack or Flush if they already existed.

The tomfoolery is a quagmire. https://groups.google.com/forum/#!topic/golang-nuts/zq_i3Hf7Nbs It leads to a combinatorial explosion of wrapper code.

From what I can tell nothing hijacks of flushes this responsewriter.

OK, let's merge it

assigned to @jacobvosmaer-gitlab

added 3 commits

• 70154cc5 - Remove log statement
• b37097c9 - 'Injecter' is a better label name
• ccfb11b3 - Group metric update calls

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added 1 commit

• c7b97add - Add tests for CountingResponseWriter

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assigned to @nick.thomas

approved this merge request

merged

mentioned in commit bc8f3615