Allow Graylog to monitor OpenSearch indices and alerts if field count is near the limit

## What?

Graylog stores messages in OpenSearch. OpenSearch, by default, has a limit of 1000 fields per index. While this can be changed it has performance implications and is not recommended.

Because graylog abstracts away OpenSearch, its not immediately clear or evident that an index is close to this limit or in danger of exceeding it.

When this limit is hit, no further messages can be indexed in the index that has reached this limit which is a critical state for a graylog cluster.

## Why?

Provide transparency to the user about the health of OpenSearch indices and help prevent reaching field limit