docs(messaging): add gcp_pubsub unary pull example

[alevenberg]

Part of #527

Changes

Please provide a brief description of the changes here.

Note: if the PR is touching an area that is not listed in the existing areas, or the area does not have sufficient domain experts coverage, the PR might be tagged as experts needed and move slowly until experts are identified.

Used make table-generation && make fix && make markdownlint && yamllint

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

Left a few minor comments.

It's mostly good to go, but the recent addition of gcp.project.id needs a bit more polishing.
Alternatively, I'm happy to approve if the introduction of the gcp.project.id is removed and done in a separate PR.

Also checking if @jsuereth wants to take a look from Google Cloud perspective.

[alevenberg]

Left a few minor comments.

It's mostly good to go, but the recent addition of gcp.project.id needs a bit more polishing. Alternatively, I'm happy to approve if the introduction of the gcp.project.id is removed and done in a separate PR.

Also checking if @jsuereth wants to take a look from Google Cloud perspective.

Thank you for all the reviews-- sorry it took so long!

I removed the project.id for now. We can add it in a separate PR.

[lmolkova]

@alevenberg thank you!

BTW, we've recently introduced messaging.operation.name attribute which you can now also use to define GCP pubsub-specific operations more formally. This is not a blocker for this PR for sure.