Add b3 metadata in grpc

[henrich-m]

What does this PR do?
It adds the support of B3 Metadata injection in gRPC calls.

Motivation
The B3 Standard is adopted in almost all observability frameworks, including dd-trace, it already does the injection and extraction in HTTP calls, but not in GRPC calls, we have 2 backends services that communicate with each other through gRPC and I can't see the traces correlated because one of them is written in Golang and utilizes Open Telemetry, that's why we need the B3 also in Metadata.

How to test the change?
Unit tests should cover the changes. :)

[marcotc]

👋 @henrich-m, this PR is super flushed out, thank you! I left a minor comment.

If I can: could you add a new configuration to the gRPC integration: option :distributed_tracing, default: true (at lib/datadog/tracing/contrib/grpc/configuration/settings.rb)? This allows anyone to disable header propagation, in case gPRC is used strictly to contact 3rd parties: in that case, you might not want to expose internal headers to them. You can see most of other HTTP (e.g. Faraday, Ethon) integrations implement such option.

[marcotc]

I think it's ok to use

          B3_HEADER_TRACE_ID = 'x-b3-traceid'.freeze
          B3_HEADER_SPAN_ID = 'x-b3-spanid'.freeze
          B3_HEADER_SAMPLED = 'x-b3-sampled'.freeze
          B3_HEADER_SINGLE = 'b3'.freeze

directly, given these are mostly likely the standard header names for most transport protocols for B3.

I see the possibility of a different protocol where these tags have to be different, but I think HTTP and gRPC can share the existing constants we already had above.

[henrich-m]

I was just following the same approach adopted for Datadod headers, I will refactor them :D

[marcotc]

Thank you, @henrich-m! 🙇

[ivoanjo]

👋 @henrich-m this change has been released in v1.2.0.
Thanks a bunch for the awesome contribution :)