metrics: Sink contructor from metric type

[codebien]

I did this multiple times across different outputs, maybe we can have a central function.

[codecov-commenter]

Codecov Report

Merging #3068 (1c5c183) into master (c00bfbb) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 1c5c183 differs from pull request most recent head 5458008. Consider uploading reports for the commit 5458008 to get more accurate results

@@            Coverage Diff             @@
##           master    #3068      +/-   ##
==========================================
+ Coverage   76.72%   76.74%   +0.02%     
==========================================
  Files         229      229              
  Lines       17110    17112       +2     
==========================================
+ Hits        13127    13133       +6     
+ Misses       3140     3135       -5     
- Partials      843      844       +1     

Flag	Coverage Δ
ubuntu	76.74% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
metrics/registry.go	94.44% <100.00%> (+2.95%)	⬆️
metrics/sink.go	95.89% <100.00%> (+0.89%)	⬆️

... and 3 files with indirect coverage changes

[mstoykov]

I have no problems with the change. I am not so certain whether the usage of the internal Sink implementation is wise in outputs.

[codebien]

https://github.com/grafana/xk6-output-prometheus-remote/blob/092f73237a7e72bc93236c280982834682702ed8/pkg/remotewrite/remotewrite.go#L374-L406

It could be now something simpler like:

newSeriesWithMeasure() {
if mt == metrics.Trend {
return CustomSinkForTrend 
}

return metrics.NewSink(mt)
}

Another case:

k6/metrics/engine/engine.go

Line 198 in db36955

sink: metrics.NewSinkByType(m.Type),

And I'm doing a new one for #1700

[oleiade]

👍🏻