feat: add type label to cortex_discarded_samples_total metric (#6221)

[ANAMASGARD]

What this PR does

Adds a type label to the cortex_discarded_samples_total metric to distinguish between float and native histogram samples.

• type="float" for regular float samples
• type="histogram" for native histogram samples

This enhancement helps operators identify whether discarded samples are floats or native histograms, improving observability when debugging data ingestion issues.

Which issue(s) this PR fixes

Fixes #6221

Changes

pkg/util/validation/validate.go

• Added SampleTypeFloat = "float" and SampleTypeHistogram = "histogram" constants
• Updated discardedSamples metric definition to include "type" label
• Updated all DiscardedSamples.WithLabelValues calls in ValidateSampleTimestamp, ValidateLabels, and ValidateNativeHistogram functions

pkg/ingester/ingester.go

• Updated all DiscardedSamples.WithLabelValues calls with appropriate sample types

pkg/distributor/distributor.go

• Updated all DiscardedSamples.WithLabelValues calls
• Split combined float+histogram counts into separate calls for accurate categorization

pkg/util/validation/validate_test.go

• Updated all test assertions to expect the new type label

CHANGELOG.md

• Added enhancement entry

Checklist

• Tests updated
• CHANGELOG.md updated
• Documentation: N/A (no new flags/configs)

Breaking Change Notice

Warning: This is a breaking change for dashboards and alerting rules that query cortex_discarded_samples_total. The metric now has an additional type label.

Migration Example

# Before
cortex_discarded_samples_total{reason="sample_out_of_bounds"}

# After (to get same behavior)
sum by (reason, user) (cortex_discarded_samples_total{reason="sample_out_of_bounds"})

# Or query specific type
cortex_discarded_samples_total{type="float"}
cortex_discarded_samples_total{type="histogram"}