chore: mark @opentelemetry/exporter-prometheus as experimental

[legendecas]

@opentelemetry/exporter-prometheus is primarily exporting metrics. If @opentelemetry/api-metrics and @opentelemetry/sdk-metrics-base are still experimental, then @opentelemetry/exporter-prometheus would better left as experimental.

[codecov]

Codecov Report

Merging #2479 (54276e0) into main (20cb74a) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2479      +/-   ##
==========================================
- Coverage   93.23%   93.20%   -0.04%     
==========================================
  Files         125      137      +12     
  Lines        4419     5017     +598     
  Branches      924     1060     +136     
==========================================
+ Hits         4120     4676     +556     
- Misses        299      341      +42     

Impacted Files	Coverage Δ
.../opentelemetry-exporter-otlp-http/src/transform.ts	88.69% <0.00%> (ø)
...lemetry-exporter-otlp-http/src/OTLPExporterBase.ts	92.15% <0.00%> (ø)
...telemetry-sdk-trace-web/src/StackContextManager.ts	94.28% <0.00%> (ø)
...kages/opentelemetry-exporter-otlp-http/src/util.ts	100.00% <0.00%> (ø)
...ry-context-zone-peer-dep/src/ZoneContextManager.ts	85.18% <0.00%> (ø)
...entelemetry-sdk-trace-web/src/WebTracerProvider.ts	100.00% <0.00%> (ø)
packages/opentelemetry-sdk-trace-web/src/utils.ts	94.90% <0.00%> (ø)
...ages/opentelemetry-exporter-otlp-http/src/types.ts	100.00% <0.00%> (ø)
...lemetry-exporter-otlp-http/src/transformMetrics.ts	95.71% <0.00%> (ø)
...es/opentelemetry-context-zone-peer-dep/src/util.ts	100.00% <0.00%> (ø)
... and 2 more

[dyladan]

I agree