refactor: Add retention hours to discarded metrics

[DylanGuedes]

What this PR does / why we need it:
We have no distinction between the type of data that gets discarded. On this PR I'm adding retention_hours to our discarded metrics so that we can better look at what data was discarded.

I'm also using a more performant form of retention strigifying when loading the retention string. Benchmark results:

goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/util
cpu: AMD Ryzen 9 7900X 12-Core Processor            
BenchmarkRetentionHours/current/24h-24     	35803432	        34.14 ns/op	       2 B/op	       1 allocs/op
BenchmarkRetentionHours/current/7d-24      	33798613	        35.33 ns/op	       3 B/op	       1 allocs/op
BenchmarkRetentionHours/current/30d-24     	23425309	        50.95 ns/op	      16 B/op	       2 allocs/op
BenchmarkRetentionHours/current/365d-24    	23175356	        51.52 ns/op	      16 B/op	       2 allocs/op
BenchmarkRetentionHours/optimized/24h-24   	412592362	         2.916 ns/op	       0 B/op	       0 allocs/op
BenchmarkRetentionHours/optimized/7d-24    	89319780	        14.32 ns/op	       3 B/op	       1 allocs/op
BenchmarkRetentionHours/optimized/30d-24   	88080440	        14.60 ns/op	       3 B/op	       1 allocs/op
BenchmarkRetentionHours/optimized/365d-24  	75923084	        15.57 ns/op	       4 B/op	       1 allocs/op
PASS
ok  	github.com/grafana/loki/v3/pkg/util	11.191s

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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[DylanGuedes]

this check is to help using this same function for when you want to automatically gets the Tenant retention but have no labels available