feat(alertmanager): Add exporter implementation

[mcube8]

Description: Implementation of Alertmanager Exporter
Span Events are exported to Prometheus Alertmanager

Link to tracking Issue: #23659

Testing: Unit tests for config, factory and alertmanager exporter implementation

Documentation: Readme and Sample Configs to use Alertmanager exporter

[linux-foundation-easycla]

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[mx-psi]

This component does not have a sponsor yet, blocking until we have a sponsor.

The PR would also need to be split, and CLA signed, but let's figure out the sponsor situation first.

[mcube8]

/easycla

[linux-foundation-easycla]

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[sokoide]

/easycla

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.