[GCP] Add GCP Custom Metricset

[legoguy1000]

What does this PR do?

Add GCP Custom 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
• Closes [GCP] Migrate gcp.metrics metricset to data stream #3718

Screenshots

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💔 Tests Failed

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Build stats

• Start Time: 2022-07-22T05:01:19.744+0000

• Duration: 19 min 33 sec

Test stats 🧪

Test	Results
Failed	1
Passed	47
Skipped	0
Total	48

Test errors

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static test: Verify sample_event.json – gcp.metrics

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 one or more errors found in document: [0] field "gcp.metrics.instance.uptime_total.value" is undefined 

Steps errors

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Test integration: gcp

• Took 4 min 28 sec . View more details here
• Description: eval "$(../../build/elastic-package stack shellinit)" ../../build/elastic-package test -v --report-format xUnit --report-output file --test-coverage

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (4/4)	💚
Files	100.0% (4/4)	💚 3.0
Classes	100.0% (4/4)	💚 3.0
Methods	92.754% (64/69)	👍 2.997
Lines	95.21% (1113/1169)	👍 3.679
Conditionals	100.0% (0/0)	💚

[endorama]

👋 @legoguy1000 there have been some internal discussion around migrating the gcp.metrics metricset, I reported the conclusions here.

Due to those considerations, I'm going to close this PR. If you want to discuss our conclusions further feel free to comment on the linked issue.

Thanks a lot for your contribution, as usual! 💪