OpenTelemetry support needs to use its own context propagator

[feywind]

Re this comment: #1389 (comment)

The Node Pub/Sub library should be using a private context propagator of type W3CTraceContextPropagator rather than the global one (which allows other libraries to stuff values that become invalid Pub/Sub attributes).

A separate issue is filed in regards to the x-cloud-trace-header that's the proximate problem here: GoogleCloudPlatform/opentelemetry-operations-js#781

(but this library should be immune to those kinds of issues, anyway...)

[aabmass]

The Node Pub/Sub library should be using a private context propagator of type W3CTraceContextPropagator rather than the global one

I feel a bit conflicted by this. If you look at PubSub as an asynchronous transport, why shouldn't the client support different propagation formats?

[feywind]

I meant the "should" there in the sense that our spec for the work included using a private, custom propagator, and it isn't, currently. But the reasoning for that is basically this exact sort of thing, we're intending to pass along the trace ID through Pub/Sub attributes, but it's not a generic transport like HTTP. Attribute names are restrictive in some cases, and (IIRC) it can cost users to add more attributes unexpectedly. Our libraries are the expected producers and consumers of this attribute, so it makes more sense to control what goes there.