allow exposing meta information for registered metrics

[hagen1778]

New public method ExposeMetadata allows enabling exposition of dummy meta-info for all exposed metrics across all Sets.

This feature is needed to improve compatibility
with 3rd-party scrapers that require meta information to be present.

This commit doesn't update exposition of default system/process metrics to keep the list of changes small. This change should be added in a follow-up commit.

#48

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (cd448dd) 85.89% compared to head (8949879) 89.30%.
Report is 7 commits behind head on master.

❗ Current head 8949879 differs from pull request most recent head 4fc879e. Consider uploading reports for the commit 4fc879e to get more accurate results

Files	Patch %	Lines
floatcounter.go	0.00%	3 Missing ⚠️
summary.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   85.89%   89.30%   +3.40%     
==========================================
  Files          11       12       +1     
  Lines         936      991      +55     
==========================================
+ Hits          804      885      +81     
+ Misses        109       76      -33     
- Partials       23       30       +7     

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[hagen1778]

@valyala one thing I didn't account for is using counters as gauges, as you do here:

	relabelConfigSuccess      = metrics.NewCounter(`vmagent_relabel_config_last_reload_successful`)

I think, this makes this PR useless when used like this.

[valyala]

@hagen1778 , thanks for the pull request!

[valyala]

one thing I didn't account for is using counters as gauges, as you do here:
relabelConfigSuccess = metrics.NewCounter(vmagent_relabel_config_last_reload_successful)
I think, this makes this PR useless when used like this.

The commit b23fdf5 adds support for Gauge.Set() method, which can be used for setting gauge value without the need to pass callback to NewGauge(). This commit is included in the tag v1.29.0. So now the gauge value can be updated with the following code:

// Create a gauge with nil callback
g := metrics.NewGauge(gaugeName, nil)

// Set newValue to the gauge
g.Set(newValue)