[receiver/elasticsearch] Request to add metric for active index merges

[crobert-1]

Component(s)

receiver/elasticsearch

Describe the issue you're reporting

This is a request to add a metric to the Elasticsearch receiver that measures the current active index merges. This would be helpful in monitoring the state of an Elasticsearch cluster. This metric could be next to the existing elasticsearch.index.operations.merge.size and elasticsearch.index.operations.merge.docs_count metrics.

[github-actions]

Pinging code owners:

• receiver/elasticsearch: @djaglowski @BinaryFissionGames

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