Metric label enrich

[hstan]

This PR is a rework on #1271 based on @jmacd 's implementation in #1421

It adds an Enricher API that supports attaching baggage attributes as metric labels.

More discussion can be found in the original PR review #1271
and #1271

[codecov]

Codecov Report

Merging #1480 (31686a0) into master (c066f15) will increase coverage by 0.0%.
The diff coverage is 95.0%.

@@          Coverage Diff           @@
##           master   #1480   +/-   ##
======================================
  Coverage    79.0%   79.0%           
======================================
  Files         128     128           
  Lines        6685    6703   +18     
======================================
+ Hits         5282    5299   +17     
- Misses       1148    1149    +1     
  Partials      255     255           

Impacted Files	Coverage Δ
sdk/export/metric/metric.go	97.3% <ø> (ø)
sdk/metric/sdk.go	81.1% <93.7%> (+0.9%)	⬆️
baggage/baggage.go	100.0% <100.0%> (ø)
sdk/metric/controller/basic/config.go	100.0% <100.0%> (ø)
sdk/metric/controller/basic/controller.go	93.3% <100.0%> (+<0.1%)	⬆️

[hstan]

@jmacd Could you please take a look when got a chance

[jmacd]

@hstan I haven't forgotten about this, but with the recent re-boot of OTel metrics taking place it's difficult to focus on enrichment. Would you be interested in attending one of the now weekly API/SDK SIG meetings? It's Thursdays on the OTel public calendar.

[hstan]

@jmacd No worries. Sure I'm more than happy to attend the meeting, could you send me an invitation? Thanks

[jmacd]

I can't figure out how to invite you -- without an e-mail address, and oddly I'm having trouble linking to an event on the OTel public calendar.

https://calendar.google.com/calendar?cid=Z29vZ2xlLmNvbV9iNzllM2U5MGo3YmJzYTJuMnA1YW41bGY2MEBncm91cC5jYWxlbmRhci5nb29nbGUuY29t

There is a meeting this week on Thursday at 4pm PST (and every two weeks), which is probably your best bet.

[hstan]

I can't figure out how to invite you -- without an e-mail address, and oddly I'm having trouble linking to an event on the OTel public calendar.

https://calendar.google.com/calendar?cid=Z29vZ2xlLmNvbV9iNzllM2U5MGo3YmJzYTJuMnA1YW41bGY2MEBncm91cC5jYWxlbmRhci5nb29nbGUuY29t

There is a meeting this week on Thursday at 4pm PST (and every two weeks), which is probably your best bet.

@jmacd I've got a team event today conflicts with the meeting, I'll try to join next time. btw my email is tanhanshuo(at)gmail.com

[MrAlias]

Closing as the metrics signal refactor at the specification level might make this work wasted. We can open again if the specification solidifies and it is determined needed.