Lightstep metrics SDK: Fix race condition in sync-delta code path

[jmacd]

Description:
There was a race in the delta-temporality collection path for synchronous instruments when multiple readers were present. This fixes that race (and a test that was validating the bad behavior).

Link to tracking Issue: Fixes #204

Testing: Lots of -count=1000 testing to reproduce locally what seems easy in the CI pipeline

[codecov-commenter]

Codecov Report

Merging #205 (4b4d1bc) into main (38d5716) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 4b4d1bc differs from pull request most recent head ce2f719. Consider uploading reports for the commit ce2f719 to get more accurate results

@@           Coverage Diff           @@
##             main     #205   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files          62       62           
  Lines        3260     3263    +3     
=======================================
+ Hits         3074     3077    +3     
  Misses        142      142           
  Partials       44       44           

Impacted Files	Coverage Δ
lightstep/sdk/metric/internal/syncstate/sync.go	100.00% <100.00%> (ø)
...htstep/sdk/metric/internal/viewstate/collectors.go	95.45% <100.00%> (+0.21%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fb8cfe2...ce2f719. Read the comment docs.