update OTEL proto files and OTLP Exporter to match the changes in the metrics specifications

[vvydier]

Creating this PR again as the last one ( #300 ) failed on EasyCLA possibly due to incorrect user.email. I am setting git config user.email "vvydier@gmail.com" before git commit this time.
Description of the PR: Labels has been deprecated in open-telemetry/opentelemetry-proto#342 and attributes are now used in it's place. This was reported by @trevor-dialpad and the OTLP Exporter needed this change if a user uses opentelemetry-swift with along other otel sdk's or uses it with newer version of the otel-collector

[codecov]

Codecov Report

Merging #301 (fda29a8) into main (1c648c1) will decrease coverage by 0.92%.
The diff coverage is 30.41%.

❗ Current head fda29a8 differs from pull request most recent head d720fa3. Consider uploading reports for the commit d720fa3 to get more accurate results

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
- Coverage   75.35%   74.42%   -0.93%     
==========================================
  Files         361      361              
  Lines       22569    22796     +227     
==========================================
- Hits        17006    16967      -39     
- Misses       5563     5829     +266     

Impacted Files	Coverage Δ
...xporters/OpenTelemetryProtocol/proto/logs.pb.swift	0.00% <0.00%> (ø)
.../OpenTelemetryProtocol/proto/logs_service.pb.swift	0.00% <ø> (ø)
...rters/OpenTelemetryProtocol/proto/metrics.pb.swift	22.48% <ø> (+2.78%)	⬆️
...enTelemetryProtocol/proto/metrics_service.pb.swift	63.15% <ø> (ø)
...ters/OpenTelemetryProtocol/proto/resource.pb.swift	90.00% <ø> (ø)
.../OpenTelemetryProtocol/proto/trace_config.pb.swift	0.00% <0.00%> (ø)
...enTelemetryProtocol/proto/trace_service.grpc.swift	84.37% <ø> (ø)
...OpenTelemetryProtocol/proto/trace_service.pb.swift	63.15% <ø> (ø)
.../OpenTelemetryProtocol/metric/MetricsAdapter.swift	31.84% <10.90%> (-1.15%)	⬇️
...orters/OpenTelemetryProtocol/proto/common.pb.swift	38.56% <36.36%> (-11.82%)	⬇️
... and 10 more

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[nachoBonafonte]

🚀

[vvydier]

Thanks @nachoBonafonte! That was easy, after almost a week of trying to figure out the problem with the PR. I will make sure to set the user.email correctly before I make a commit next time!