fix: Pass immutable metrics to PushMetricExporter

[cijothomas]

This is related to #2921, but a smaller scoped change - this modifies exporter trait so that exporters get a immutable reference to metrics. There is not requirement to pass a mutable reference to exporters.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.3%. Comparing base (1d610a2) to head (456fcf1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/exporter/http/mod.rs	0.0%	2 Missing ⚠️
opentelemetry-otlp/src/exporter/tonic/metrics.rs	0.0%	2 Missing ⚠️
opentelemetry-otlp/src/exporter/http/metrics.rs	0.0%	1 Missing ⚠️
opentelemetry-otlp/src/metric.rs	0.0%	1 Missing ⚠️
.../src/metrics/periodic_reader_with_async_runtime.rs	0.0%	1 Missing ⚠️
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2947     +/-   ##
=======================================
- Coverage   81.3%   81.3%   -0.1%     
=======================================
  Files        126     126             
  Lines      24294   24294             
=======================================
- Hits       19775   19774      -1     
- Misses      4519    4520      +1     

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