Support decoding optional headers in the OTLP-Arrow receiver

[jmacd]

Description:

This sets the incoming gRPC headers from the new optional bytes field added here: f5/otel-arrow-adapter#85. The behavior of this change matches #35 in the exporter.

The gRPC stream's incoming context is merged with the per-request metadata calculated for the individual data item. This addresses a problem described in greater detail in open-telemetry/opentelemetry-collector#6965. There is a suggestion that OTC would be better off with a new extension type for non-auth-related metadata, but for now the Arrow receiver expects the exporter to run the Auth extension on the request context and serialize the result; the best the receiver can do is merge the two metadatas.

Part of #33

[lquerel]

LGTM

[jmacd]

Updated the receiver to use client.Info.Metadata which is the Collector's standard mechanism for receivers to convey headers. PTAL especially @paivagustavo, thanks.

[jmacd]

Note: I've updated this PR with a slight fix. This code now respects the GRPCServerSettings.IncludeMetadata flag. This must be set for headers to be set into the client context.

[jmacd]

Due to the changes in #35 I will now update this PR to merge the incoming context (from the stream, including the static metadata and any the Auth plugin returns for a background context) with the per-request metadata that is transmitted per batch by the exporter.

[jmacd]

This PR now matches #35: when IncludeMetadata is set the stream context's gRPC incoming metadata is merged with the per-request context. this means the context constructed by the OTLP-Arrow receiver contains the results of evaluating the Auth extension twice, once on the background context and once on the per-request context. PTAL.

[jmacd]

Did some refactoring of the new header-decoding logic to make it more readable, added a few tests and corrected lint errors. 88f4210