DOCS: use lower case header

[v1v]

Motivation/summary

If I use the lowercase header then it does work otherwise I get an error:

E0427 12:07:17.398505136    3317 call.cc:919]                validate_metadata: {"created":"@1619525237.398491209","description":"Illegal header key","file":"src/core/lib/surface/validate_metadata.cc","file_line":44,"offset":0,"raw_bytes":"41 75 74 68 6f 72 69 7a 61 74 69 6f 6e 'Authorization'\u0000"}
Exception while exporting Span batch.
Traceback (most recent call last):
  File "/tmp/workspace/ansible-plugin/.venv/lib/python3.8/site-packages/opentelemetry/sdk/trace/export/__init__.py", line 333, in _export_batch
    self.span_exporter.export(self.spans_list[:idx])  # type: ignore
  File "/tmp/workspace/ansible-plugin/.venv/lib/python3.8/site-packages/opentelemetry/exporter/otlp/proto/grpc/trace_exporter/__init__.py", line 294, in export
    return self._export(spans)
  File "/tmp/workspace/ansible-plugin/.venv/lib/python3.8/site-packages/opentelemetry/exporter/otlp/proto/grpc/exporter.py", line 261, in _export
    self._client.Export(
  File "/tmp/workspace/ansible-plugin/.venv/lib/python3.8/site-packages/grpc/_channel.py", line 944, in __call__
    state, call, = self._blocking(request, timeout, metadata, credentials,
  File "/tmp/workspace/ansible-plugin/.venv/lib/python3.8/site-packages/grpc/_channel.py", line 926, in _blocking
    call = self._channel.segregated_call(
  File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 498, in grpc._cython.cygrpc.Channel.segregated_call
  File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 366, in grpc._cython.cygrpc._segregated_call
  File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 360, in grpc._cython.cygrpc._segregated_call
  File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 218, in grpc._cython.cygrpc._call
  File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 253, in grpc._cython.cygrpc._call
  File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 61, in grpc._cython.cygrpc._raise_call_error
ValueError: metadata was invalid: ('Authorization', 'Bearer ****')

The above error happened with the below python OTEL dependencies:

• opentelemetry-api
• opentelemetry-sdk
• opentelemetry-exporter-otlp

Installing collected packages: six, protobuf, opentelemetry-semantic-conventions, opentelemetry-api, opentelemetry-sdk, opentelemetry-proto, grpcio, googleapis-common-protos, backoff, opentelemetry-exporter-otlp-proto-grpc, opentelemetry-exporter-otlp
Successfully installed backoff-1.10.0 googleapis-common-protos-1.53.0 grpcio-1.37.0 opentelemetry-api-1.1.0 opentelemetry-exporter-otlp-1.1.0 opentelemetry-exporter-otlp-proto-grpc-1.1.0 opentelemetry-proto-1.1.0 opentelemetry-sdk-1.1.0 opentelemetry-semantic-conventions-0.20b0 protobuf-3.15.8 six-1.15.

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• Build Cause: Pull request #5169 updated

• Start Time: 2021-05-13T11:30:54.324+0000

• Duration: 40 min 45 sec

• Commit: 484ea90

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Total	6414

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[axw]

LGTM. I think it might be worth opening an issue on opentelemetry-python, seems to me they should be lowercasing the headers.

[v1v]

I just reported open-telemetry/opentelemetry-python#1851

[eddyleelin]

Shouldn't this update also apply to the description of OTEL_EXPORTER_OTLP_HEADERS right under where you had made the change in the doc?

This line
Authorization header that includes the Elastic APM Secret token or API key: "Authorization=ApiKey api_key".
Should then be changed to
Authorization header that includes the Elastic APM Secret token or API key: "authorization=ApiKey api_key".

https://github.com/v1v/apm-server/blob/484ea907ed9ac848b8af0e5b64dfc13452ccbb9e/docs/guide/opentelemetry-elastic.asciidoc#instrument-applications-1