feat(otlp-transformer)!: accept ResourceMetrics instead of ResoruceMetrics[] in metrics serializers

[pichlermarc]

Which problem is this PR solving?

See #3276 - the old exporter bases did not support non-array export types so we worked around it by having a proxy exporter class that took ResourceMetrics[] that does not match the PushMetricExporter interface (which requires ResourceMetrics), and a base implementation that matches the PushMetricExporter interface and converts the ResourceMetrics into ResourceMetrics[]. As the issue linked above states, this was not optimal and looked quite confusing to most.

However, with recent changes in #5031, #4542, #4971, #4895 and #4743, we don't need this workaround anymore and can now use a more direct approach.

This PR accomplishes this by aligining serializer to take ResourceMetricsas provided by the PushMetricExporter interface instead of ResourceMetrics[] and delegates wrapping ResourceMetrics to the ISerializer implementation.

Fixes #3276

Breaking changes

• (user-facing): ProtobufMetricsSerializer now only accepts ResourceMetrics instead of ResourceMetrics[] to align with PushMetricExporter requirements
• (user-facing): JsonMetricsSerializer now only accepts ResourceMetrics instead of ResourceMetrics[] to align with PushMetricExporter requirements

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Existing tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.55%. Comparing base (2a5e0d8) to head (adf81b1).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5159      +/-   ##
==========================================
- Coverage   94.57%   94.55%   -0.02%     
==========================================
  Files         314      314              
  Lines        7961     7956       -5     
  Branches     1600     1600              
==========================================
- Hits         7529     7523       -6     
- Misses        432      433       +1     

Files with missing lines	Coverage Δ
...er-metrics-otlp-http/src/OTLPMetricExporterBase.ts	89.06% <100.00%> (-0.80%)	⬇️
.../packages/otlp-transformer/src/json/serializers.ts	100.00% <100.00%> (ø)
...kages/otlp-transformer/src/protobuf/serializers.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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