feat(pyroscope.write): Add labels validation

[marcsanmi]

PR Description

This PR adds validation for labels in the pyroscope.write component to ensure compatibility with Pyroscope's validation requirements. The validation:

• Checks for duplicate labels
• Validates label names & values according to Pyroscope's standards

Which issue(s) this PR fixes

https://github.com/grafana/pyroscope-squad/issues/357

Notes to the Reviewer

• I don't check for service_name label existence. As talked, as far as using the receive_http, this should come for free with our ensureServiceName function and also validated at pyroscope side.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[simonswine]

Thanks for adding this.

LGTM

[simonswine]

Nit: Find that easier to read and less error prone

Suggested change

	// Skip label name validation for pyroscope reserved labels
	if l.Name == pyroscope.LabelName {
	lastLabelName = l.Name
	continue
	}
	// Validate label name
	if err := labelset.ValidateLabelName(l.Name); err != nil {
	return fmt.Errorf("invalid label name: %w", err)
	}
	// Skip label name validation for pyroscope reserved labels
	if l.Name != pyroscope.LabelName {
	// Validate label name
	if err := labelset.ValidateLabelName(l.Name); err != nil {
	return fmt.Errorf("invalid label name: %w", err)
	}
	}

[simonswine]

If you disagree, just ignore

[simonswine]

LGTM