Added PerRPCCredentials for gRPC settings

[jpkrohling]

Signed-off-by: Juraci Paixão Kröhling juraci@kroehling.de

Description:
This PR adds support for including a per-RPC authentication to gRPC settings. Initially, only "bearer" token has been added, but can be easily extended in the future based on real-world needs. The token can be read directly from the configuration file or from an external token file, such as the ones injected by Kubernetes into pods.

Link to tracking Issue: n/a

Testing: this was successfully tested with a custom processor, reading the bearer token from the context via metadata.FromIncomingContext(ctx). Additionally, unit tests were added to this PR.

Documentation: the config.yaml reference file has been updated to include this new option.

[jpkrohling]

For a bit more context: I'm building a PoC for a multi-tenant solution, where the agent reads the token from the file system and adds it as part of the RPC payload to the remote collector. The collector then extracts the tenant information based on the JWT and enhances the spans with a tenant resource value.

The feature in this PR is quite generic and would be useful to quite a good number of use cases, not only in the scenario I described above.

The processor I mentioned before still needs some work, but will be contributed to the opentelemetry-collector-contrib in the next few days, once this PR here gets accepted (as this is a requirement for the processor).

[codecov]

Codecov Report

Merging #1250 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1250   +/-   ##
=======================================
  Coverage   89.73%   89.74%           
=======================================
  Files         215      216    +1     
  Lines       15136    15150   +14     
=======================================
+ Hits        13583    13597   +14     
  Misses       1134     1134           
  Partials      419      419           

Impacted Files	Coverage Δ
config/configgrpc/bearer_token.go	100.00% <100.00%> (ø)
config/configgrpc/configgrpc.go	100.00% <100.00%> (ø)
translator/internaldata/resource_to_oc.go	83.72% <0.00%> (ø)

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[pjanotti]

@jpkrohling could you please add code (and test) to handle the case when the auth type is unknown other than that LGTM.

[jpkrohling]

PR updated. The only point to clarify seems to be regarding the ability to read tokens from local files.

(PS: for some reason, I can't seem to be able to re-request a review from @odeke-em)

[pjanotti]

Please, fix the error msg in case of an unknown auth type.