Add monitoring and alerts for embeddings indexer

Background

Embeddings were added to the index in #451431 (closed)

Proposal

1. Add prometheus metrics to expose the queue size for the embeddings cron worker

The metrics are emitted in the Elastic::MetricsUpdateService

2. Consume new metrics and generate alerts when the queue is backed up.

The existing alerts are defined in https://gitlab.com/gitlab-com/runbooks and for production and staging. The alerts are fired when the queue reaches 10_000 for staging and 50_000 for production