ignore phpstan false positive on metrics multiobserver

[brettmc]

metrics observer callbacks can be removed mid-flight, so ignore the false positive

[codecov]

Codecov Report

Merging #813 (dc63b22) into main (77838b3) will increase coverage by 1.85%.
The diff coverage is 100.00%.

❗ Current head dc63b22 differs from pull request most recent head 783768c. Consider uploading reports for the commit 783768c to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #813      +/-   ##
============================================
+ Coverage     81.03%   82.88%   +1.85%     
- Complexity     1825     1829       +4     
============================================
  Files           225      225              
  Lines          4687     4697      +10     
============================================
+ Hits           3798     3893      +95     
+ Misses          889      804      -85     

Flag	Coverage Δ
7.4	82.89% <100.00%> (+1.86%)	⬆️
8.0	82.91% <100.00%> (+1.85%)	⬆️
8.1	82.91% <100.00%> (+1.85%)	⬆️

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Impacted Files	Coverage Δ
src/Contrib/OtlpGrpc/Exporter.php	91.42% <100.00%> (+1.26%)	⬆️
src/SDK/Metrics/MetricObserver/MultiObserver.php	100.00% <100.00%> (ø)
src/SDK/Trace/Behavior/HttpSpanExporterTrait.php	93.61% <100.00%> (+0.13%)	⬆️
src/SDK/Trace/Span.php	94.23% <0.00%> (+81.73%)	⬆️

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