Incorrect `es_data_node` label value for specialized data nodes

[dcupif]

Hi, I've encountered a surprising behaviour where the exported metrics for specialized data nodes will report es_data_node=false.

Looking at the code, it will only report es_data_node=true if the node has the role data. Is this expected?

Shouldn't the exporter report es_data_node=true if the node has a role within data.*?

[giray]

elasticsearch_filesystem_data_available_bytes{es_data_node="true"} currently returns zero results

[zdlouhy]

Hi, same issue on elasticsearch v8.11.0

[sysadmind]

Two things would be helpful in fixing this bug:

1. The output of /_nodes/stats so that we can include it in our tests.
2. Any links to documentation about the values that can be expected