Prometheus ipfs_http_request metrics are not exposed

[thibmeu]

Version information:

Custom implementation not using the cli
go-ipfs version: v0.8.0 tag
System version: amd64/linux
Golang version: 1.15.8

Description:

Starting v0.8.0, prometheus metrics exposed by MetricsCollectionOption are not exposed to prometheus DefaultRegisterer.
When OpenCensus Metrics were added, all prometheus.Register of MetricsCollectionOption were changed to register on a new custom registry. There are no comment on this change in the PR or in the changelog for v0.8.0.

// file core/corehttp/metrics.go
promRegistry := prometheus.NewRegistry()
//...

if err := promRegistry.Register(reqCnt); err != nil {
  // note: this is always false, as the registry is new.
  if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
//...

This registry is not returned and does not include any collector. So while metrics are still registered, I cannot find how to expose them.

A possible fix is to remove the addition of this new registry for this metric. An other option is to expose this new registry as a global variable which can then be collected.

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[Lekensteyn]

CC @lanzafame @aschmahmann, a potential regression from #7593

[aschmahmann]

cc @marten-seemann if you have time to take a look

[lanzafame]

Sorry, this one is on me. #7986 contains the fix.
CC @aschmahmann

[thibmeu]

The fix looks good to me. That's the exact same one I have on my configuration.