[+] add Total index size panel to Health Check dashboard

[0xgouda]

• Add indexes_size_b column to db_size metric
  • The column returns the total index size aggregated from pg_stat_user_indexes
• Add new Total index size panel to Health Check dashboard

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• This is the first PR in a series of PRs that aim to improve the index dashboards
• I think the Health Check dashboard panels need to be re-arranged in a more logical grouping row-by-row, putting relevant panels together, but this is to be done in a separate PR

Screenshots:

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[coveralls]

Pull Request Test Coverage Report for Build 20780733018

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.573%

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Totals
Change from base Build 20746606366:	0.0%
Covered Lines:	3971
Relevant Lines:	5325

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💛 - Coveralls

[pashagolub]

Looks cool!