Upgrade go.opentelemetry.io/proto to v0.15.0

[MrAlias]

Fix #2747

[codecov]

Codecov Report

Merging #2748 (05ee0b6) into main (bbcdc75) will increase coverage by 0.0%.
The diff coverage is 63.1%.

@@          Coverage Diff          @@
##            main   #2748   +/-   ##
=====================================
  Coverage   76.7%   76.8%           
=====================================
  Files        181     181           
  Lines      12175   12174    -1     
=====================================
+ Hits        9347    9350    +3     
+ Misses      2603    2599    -4     
  Partials     225     225           

Impacted Files	Coverage Δ
...lp/otlpmetric/internal/otlpmetrictest/collector.go	0.0% <0.0%> (ø)
...otlp/otlptrace/internal/otlptracetest/collector.go	0.0% <0.0%> (ø)
.../otlp/otlptrace/internal/otlptracetest/otlptest.go	0.0% <0.0%> (ø)
...ers/otlp/otlptrace/internal/tracetransform/span.go	96.5% <92.8%> (ø)
...otlp/otlpmetric/internal/metrictransform/metric.go	78.1% <100.0%> (+<0.1%)	⬆️
...lptrace/internal/tracetransform/instrumentation.go	100.0% <100.0%> (ø)
sdk/trace/batch_span_processor.go	82.1% <0.0%> (+0.9%)	⬆️

[bgranetzke]

@MrAlias I think this PR fixed a compile-time bug in metrics/0.23.0-current(0.28.0)
https://github.com/open-telemetry/opentelemetry-go/pull/2748/files#diff-c1ccbe4e557a5f58361fbcc64ecbd6c1efeb90b5a10659113ffe25cc0cd9d81bR428

Do we need to also bump up the metrics* tags for this release?