pubsub: Add OpenTelemetry tracing

[quartzmo]

OpenTelemetry should be added to trace a message throughout its lifecycle. It should be an optional dependency that creates a trace that tracks a message from when it is published to when it is received by a subscriber. OpenTelemetry tracing should not interfere with any underlying client functions.

See googleapis/google-cloud-go/pull/5034.

See also some older work: Python PR and Node publisher impl for the state of implementations in other clients. At this time, none is complete to my knowledge.

The opentelemetry-ruby opentelemetry-api gem is 1.0.0.rc3, stable enough to begin work.

See also the opentelemetry-specification messaging samples and w3.org spec.

[quartzmo]

See also googleapis/google-api-ruby-client#3414 for an example Ruby implementation.