[FEATURE] Add node_name label to elasticsearch_indices_shards_docs metric

[andyba2016]

Hi Team:

Feature Request

Currently, the elasticsearch_indices_shards_docs metric only exposes labels for index, shard, and state.
It would be very useful to also include the node_name (or node_id) as a label, so that we can directly
correlate shard-level metrics with the node where the shard is located.

Why?

• Allows PromQL queries joining shard-level metrics with node metrics.
• Makes it easier to detect imbalances or issues on specific nodes without additional exporters.

Possible implementation

• Extend the metric descriptor in collector/indices.go to include node_name.
• Use _nodes API to map node_id → node_name.
• Alternatively, expose a new metric (elasticsearch_indices_shards_docs_by_node) to avoid breaking existing queries.

Thanks!