Add measurement processor support

[kristinapathak]

Description: Adds a MeasurementProcessor interface, as a prototype for a potential OTel specification and as a necessary feature for Lightstep's internal use.

[codecov-commenter]

Codecov Report

Patch coverage: 85.71% and no project coverage change.

Comparison is base (dc5bdc6) 89.39% compared to head (ee74846) 89.40%.

❗ Current head ee74846 differs from pull request most recent head 76ab7f1. Consider uploading reports for the commit 76ab7f1 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #493   +/-   ##
=======================================
  Coverage   89.39%   89.40%           
=======================================
  Files          62       62           
  Lines        3592     3595    +3     
=======================================
+ Hits         3211     3214    +3     
  Misses        287      287           
  Partials       94       94           

Impacted Files	Coverage Δ
lightstep/sdk/metric/sdkinstrument/performance.go	100.00% <ø> (ø)
lightstep/sdk/metric/internal/syncstate/sync.go	96.77% <85.71%> (+0.10%)	⬆️

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[jmacd]

👍