[+] add google cloud pub/sub receiver

[0xgouda]

• A gRPC receiver that writes metrics received from pgwatch
  to Google cloud pub/sub servers.

  • users can then configure different subscribers to receive data from pgwatch topic in pub/sub (e.g. BigQuery, Cloud Storage, ...etc)
  • In pub/sub data can be pulled periodically in batches or pushed directly when they arrive.

• The receiver creates a new topic called pgwatch under the provided GCP project.

• The receiver uses the official pub/sub package for golang which supports Authentication via Application Default Credentials (ADC)

[coveralls]

Pull Request Test Coverage Report for Build 17146886782

Details

• 24 of 42 (57.14%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 52.04%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/gcp_pubsub_receiver/pubsub_receiver.go	24	30	80.0%
cmd/gcp_pubsub_receiver/main.go	0	12	0.0%

Totals
Change from base Build 17079401066:	0.1%
Covered Lines:	829
Relevant Lines:	1593

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💛 - Coveralls