[exporter/alertmanager] Alerting from metrics

[cabrinha]

Component(s)

exporter/alertmanager

Is your feature request related to a problem? Please describe.

Without Prometheus, there is no way to alert using Alertmanager with OTEL metrics.

Describe the solution you'd like

I'd like OTEL to be able to send metrics into Alertmanager so that alerts can be created.

Describe alternatives you've considered

OTEL => Prom => Alertmanager, but I want to stop running Prometheus.

Additional context

This request currently exists for logs here: #37612

[github-actions]

Pinging code owners:

• exporter/alertmanager: @sokoide @mcube8

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