gcp/observability: update stackdriver dependency to remove dep on prometheus

[dfawley]

Stackdriver was never v1.0 and used a non-v1.0 version of prometheus, which moved/removed a package stackdriver was using. Stackdriver removed this dependency later, but the project is archived and didn't perform a release after removing the dependency, so we need to manually update it to an unreleased version.

cc @dashpole as FYI

RELEASE NOTES: none

[codecov]

Codecov Report

Merging #7008 (19bf50a) into master (27c5d98) will increase coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 19bf50a differs from pull request most recent head 93c8d43. Consider uploading reports for the commit 93c8d43 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7008      +/-   ##
==========================================
+ Coverage   82.28%   82.32%   +0.04%     
==========================================
  Files         296      296              
  Lines       31464    31464              
==========================================
+ Hits        25890    25904      +14     
+ Misses       4503     4490      -13     
+ Partials     1071     1070       -1     

see 17 files with indirect coverage changes