Adding with_attributes context propagation to pg instrumentation?

[yoheyk]

I'd like to have the with_attributes context propagation in the pg instrumentation also.
Currently, we have it in Redis, HTTP::ClientContext and mysql2: #1175
It's going to be the forth one so still under the threshold of duplication 😄 : #1157 (comment)
How does this sound?

[ahayworth]

@yoheyk I think we would welcome such a PR. 😄

[yoheyk]

will do!