[GCP] Add GCP PubSub metricset

[legoguy1000]

What does this PR do?

Add GCP PubSub Metricset

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates Migrate some Metricbeat modules to GCP package #490
• Requires [GCP] Add GCP Compute Metricset #2301

Screenshots

[elasticmachine]

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• Reason: The PR is not allowed to run in the CI yet

• Start Time: 2021-12-02T23:50:31.947+0000

• Duration: 2 min 14 sec

• Commit: 2bbc0c0

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