Update GCP docs with metrics data stream

[endorama]

What does this PR do?

Updates main README with metrics data streams and bump to 2.15.2.

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

• [ ]

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• Start Time: 2023-01-11T10:58:26.682+0000

• Duration: 19 min 47 sec

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Test	Results
Failed	0
Passed	53
Skipped	0
Total	53

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[kaiyan-sheng]

link: https://github.com/elastic/integrations/pull/4729

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (6/6)	💚
Files	100.0% (6/6)	💚
Classes	100.0% (6/6)	💚
Methods	90.291% (93/103)	👎 -9.709
Lines	95.976% (1455/1516)	👎 -0.987
Conditionals	100.0% (0/0)	💚

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Package gcp - 2.15.2 containing this change is available at https://epr.elastic.co/search?package=gcp