Solr index merge metrics api

[Wei]

Hi,

I am trying to check if there is solr index merge in progress.

Referring to https://solr.apache.org/guide/8_4/metrics-reporting.html

I use the API   http://localhost:8983
<http://localhost:8983/solr/collection/select>
/solr/admin/metrics?prefix=INDEX.merge

but there is nothing reported in the response:

{"responseHeader": {"status": 0,"QTime": 1},"metrics": {}}

Is there any specific config needed to enable the merge metrics?

Many thanks,
Wei