gRPC plugin throws an error when no traceparent is present

[jordanworner]

What version of OpenTelemetry are you using?

0.5.1

What version of Node are you using?

v12.16.0

What did you do?

Sent a gRPC request with no metadata

What did you expect to see?

The request completing successfully

What did you see instead?

TypeError: Cannot read property 'match' of undefined
at parseTraceParent (node_modules/@opentelemetry/core/build/src/context/propagation/HttpTraceContext.js:36:31)
at HttpTraceContext.extract (node_modules/@opentelemetry/core/build/src/context/propagation/HttpTraceContext.js:73:29)
at PropagationAPI.extract (node_modules/@opentelemetry/api/build/src/api/propagation.js:63:33)
at Object.func (node_modules/@opentelemetry/plugin-grpc/build/src/grpc.js:86:62)
at node_modules/grpc/src/server.js:593:13

Additional context

It looks like the issue is with the getter function the grpc plugin is passing to the propagation API. The grpc metadata object returns an empty array when the key isn't present, the propagator classes look like they are expecting a falsy value.