feat: add indexing pressure

[mingregister]

So far, I have not seen the indexing_pressure metrics in the elasticsearch_exporter, so i add it. #638

[mingregister]

@SuperQ hello, Could you help to review this pr. I use this pr in our production, and everything is fine,

• elasticsearch7.10 collects all metrics successfully
• elasticsearch7.7 does not have the indexing_pressure metrics but everything else works fine except can not collect the indexing_pressue metric.

Please let me know if you have any questions or concerns.

[SuperQ]

This seems like a composite metric that is the sum of the current_coordinating_in_bytes and current_primary_in_bytes metrics.

We typically drop these things to save space and cardinality when you can use foo + bar in PromQL.

[mingregister]

thanks for your review. After I check the document, current_combined_coordinating_and_primary_in_bytes or total_combined_coordinating_and_primary_in_bytes is not equal to coordinating + primary. So maybe we should keep this? Here is the document:

# https://www.elastic.co/guide/en/elasticsearch/reference/current/cluster-nodes-stats.html#cluster-nodes-stats-api-response-body-indexing-pressure
combined_coordinating_and_primary_in_bytes: 
(integer) Memory consumed, in bytes, by indexing requests in the coordinating or primary stage. This value is not the sum of coordinating and primary as a node can reuse the coordinating memory if the primary stage is executed locally.

Please let me known if you have any questions or concern.