OpenTelemetry support needs to use its own context propagator #2005
Description
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...)