feat: otlp-grpc exporter uses headers environment variables

[vreynolds]

Which problem is this PR solving?

• OTLP-gRPC exporter needs to honor the OTEL_EXPORTER_OTLP(*)HEADERS env var
• from spec:

Key-value pairs to be used as headers associated with gRPC or HTTP requests.

• fixes OTEL_EXPORTER_OTLP_HEADERS not honored by gRPC exporter #2303

Short description of the changes

• Merge headers into gRPC exporters' metadata

[codecov]

Codecov Report

Merging #2304 (73dedb9) into main (8f2209f) will not change coverage.
The diff coverage is n/a.

❗ Current head 73dedb9 differs from pull request most recent head e46afa4. Consider uploading reports for the commit e46afa4 to get more accurate results

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  Misses        376      376