Add Index Bloat overview panel to Prometheus dashboards

[Mohamed-Elfardy]

Fixes: #1166

This PR adds an Index Bloat Overview section to the Tables dashboard.

Introduces a new row for index bloat metrics
Adds a table panel showing top 10 bloated indexes by bloat ratio in tables-overview-prometheus

[coveralls]

Pull Request Test Coverage Report for Build 21809742577

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 61 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.1%) to 75.782%

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Files with Coverage Reduction	New Missed Lines	%
internal/sinks/prometheus.go	61	39.51%

Totals
Change from base Build 21722664308:	-1.1%
Covered Lines:	4118
Relevant Lines:	5434

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💛 - Coveralls

[0xgouda]

Thanks for your contribution, but please ask before working on issues if it's available to save time for both of us.

1. The problem isn't a dashboarding problem; we don't have a metric that measures index bloat.
2. The problem isn't for prom dashboards only
3. There is no index_used_size_b field in the index_stats metric (you are using it in your promQL query)

[Mohamed-Elfardy]

Thanks for the clarification, I will make sure to discuss first.