stackdriver: avoid calling log from inside exporter

[djc]

Rebased #705.

[codecov]

Codecov Report

Merging #709 (7035862) into main (6d07ff0) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #709      +/-   ##
==========================================
+ Coverage   71.36%   71.38%   +0.01%     
==========================================
  Files         101      101              
  Lines        8557     8557              
==========================================
+ Hits         6107     6108       +1     
+ Misses       2450     2449       -1     

Impacted Files	Coverage Δ
opentelemetry/src/sdk/metrics/controllers/push.rs	83.33% <0.00%> (+3.33%)	⬆️

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