GRPCPropagator: handle metadata array values

[mdehoog]

gRPC metadata values can be arrays (see https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md). In gRPC's Ruby metadata implementation, single metadata values are a singular type, and multiple values come as an array.

This library assumes a single value each time. This PR fixes this assumption to take the first value of an array, like the Go version.

This bug surfaced a separate bug with dd-trace-go, which appends values to metadata rather than setting. If your trace looks like Go -> Go -> Ruby, the gRPC call to Ruby will contain the headers twice.

[ericmustin]

👋 Thanks for the contribution @mdehoog , much appreciated.

Overall this looks great and should help us be more robust. Left some small nit comments around simply checking for an array vs coercing things into a newly created array every invocation of the relevant trace/span/priority/origin extract methods. I think should should help performance wise as we avoid naively creating an Array every time.

Additionally if possible let's try to prevent a regression here, would you mind adding a unit test to /spec/ddtrace/propagation/grpc_propagator_spec.rb ? Feel free to add any cases you think are relevant but I think simply appending a simple additional test to the describe .extract block would suffice.

dd-trace-rb/spec/ddtrace/propagation/grpc_propagator_spec.rb

Line 60 in fbb068a

describe '.extract' do

feel free to use the following or amend how you see fit.

    # Metadata values can also be arrays
    # https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md
    context 'given populated metadata in array format' do
      let(:metadata) do
        { 'x-datadog-trace-id' => ['1234567890'],
          'x-datadog-parent-id' => ['9876543210'],
          'x-datadog-sampling-priority' => ['0'],
          'x-datadog-origin' => ['synthetics'] }
      end

      it 'returns a populated context' do
        expect(subject.trace_id).to eq 1234567890
        expect(subject.span_id).to eq 9876543210
        expect(subject.sampling_priority).to be_zero
        expect(subject.origin).to eq 'synthetics'
      end
    end

[ericmustin]

🚀

[ericmustin]

@mdehoog ty again for the contribution! will ping when this is released in the next version.

[mdehoog]

thanks for the rapid merge and review!

[ericmustin]

@mdehoog we've release 0.42 with this work, if you want to ugrade https://github.com/DataDog/dd-trace-rb/releases/tag/v0.42.0