support metric_relabel for __name__

[mtanda]

What was the expected result?

Prometheus drop metrics which match pattern.

    metric_relabel_configs:
    - source_labels: [__name__]
      regex: ^foo_.*$
      action: drop

What happened instead?

Prometheus can't drop the metrics.

[mtanda]

@jkohen Would you check this?

[jkohen]

I can reproduce, I will work on a fix. Thanks!

[mtanda]

@jkohen Thanks for quick reply and fix. I close this PR :-)

[jkohen]

The PR hasn't been merged, yet, please don't close the issue.

[mtanda]

Oops, sorry. This is a PR. I should make an Issue first... Anyway, I keep this PR until merging your PR.

[jkohen]

Indeed! I betrayed my lack of fluency in GitHub. Thanks for the PR 😄 mine has tests and fixes one issue with this one, so I'll keep that.

[jkohen]

Release 0.4.2 includes the fix.

[mtanda]

Thanks lot!!