docs: adding pubsub emulator example

[maitrimangal]

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Fixes #<issue_number_goes_here> ☕️

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/pubsub/UsePubSubEmulatorExample.java:40, tag pubsub_use_emulator

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[hannahrogers-google]

LGTM, but please get approval from Tianzi as well

[anguillanneuf]

thank you. i'd recommend adding a test as well. feel free to create a new issue to track that work. you can use Cloud Build with a public container image with Pub/Sub emulator preinstalled to write the test.

cloudbuild.yaml example:

steps:
- name: gcr.io/google.com/cloudsdktool/cloud-sdk:433.0.1-emulators
   entrypoint: 'gcloud'
   args: ['beta', 'emulators', 'pubsub', 'start', '--project=abc', '--host-port=0.0.0.0:8085']
   env:
   - 'PUBSUB_EMULATOR_HOST=localhost:8085'

Also, don't forget to update docs or ping Moi Borah to update docs. :-)