Add loki.enrich component

[v-zhuravlev]

PR Description

This PR adds a new loki.enrich component that enriches Loki logs with labels from discovery.* components.
The component matches a label from incoming logs against a label from discovered targets, and copies specified labels from the matched target to the log entry.

Features:

• Copy selected labels from matched targets to logs
• Support for any discovery component (file, DNS, HTTP, etc.)

Example use cases to ensure bets logs-metrics correlation:

• Enrich Network devices syslog messages with device metadata(as labels) from IP address management (IPAM)/Data center infrastructure management(DCIM) like Netbox (https://github.com/FlxPeters/netbox-plugin-prometheus-sd). It could be same labels as used by prometheus.exporter.snmp or other metric scrapers.
• Ensure metrics and logs use same labelset (gathered from same discovery.* component) even if metrics are scraped from prometheus endpoints and logs are received via loki.source.api.

Notes to the Reviewer

• loki.enrich forwards logs unchanged if no match is found
• loki.enrich should work with any discovery.* component. Best combined with discovery.relabel to rename hidden labels if present.
• The integration test demonstrates:
  • Sample log pushed via HTTP API and then additional labels added using file-based discovery sample

PR Checklist

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

[github-actions]

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[v-zhuravlev]

Similar functionality, but not applicable to syslog as of now: https://grafana.com/docs/alloy/latest/reference/stdlib/array/#arraycombine_maps