Support Micrometer Tracing for PubSub

[wleese]

Is your feature request related to a problem? Please describe.

We provide a framework based on Spring Boot, and export our traces to Google Cloud Trace using an OpenTelemetry tracer implementation.

Looking at the current implementation of tracing for Pub/Sub in Spring Cloud GCP, we see that it is tied to Brave without any way to use our own Tracer implementation.

Preferably the tracing implementation for Pub/Sub would be implemented using Micrometer Tracing facade, so that both OpenTelemetry and Brave are supported, and would also better align Spring Cloud GCP with Spring Boot where Micrometer Tracing is already used.