Expose node stats indexing pressure

[hartfordfive]

Currently, the exporter doesn't expose stats regarding the indexing pressure. This information would be quite valuable when it comes to having the ability to automatically detect when one ore many instances are unable to accept indexing requests and consequently returning 429 errors. These statics can be obtained from the Node Stats API, specifically with indexing_pressure metric.

For example, some of these useful resulting metrics would include:

• nodes.<NODE_ID>.indexing_pressure.memory.total.primary_rejections
• nodes.<NODE_ID>.indexing_pressure.memory.total.replica_rejections

I haven't considered the most appropriate structure with regards to labels although it should be mostly similar to the logic used in the current nodes info collector. You could potentially go one step further and have a configuration flag to enable each possible metric exposed by the node stats API.

[hartfordfive]

If the repo maintainers are OK with it, I would be glad to help on contributing the code for this additional collector?