How to rename label within a metric in Prometheus

you can use the label_replace function in promQL, but it also add the label, don't replace it

label_replace(
  <vector_expr>, "<desired_label>", "$1", "<existing_label>", "(.+)"
)

label_replace(
node_systemd_unit_state{instance="server-01",job="node-exporters",name="kubelet.service",state="active"},
"unit_name","$1","name", "(.+)"
)

So, to avoid the repetition you can add:

sum(label_replace(
    node_systemd_unit_state{instance="server-01",job="node-exporters",name="kubelet.service",state="active"},
    "unit_name","$1","name", "(.+)"
    )
)by(unit_name)

I got tired of all the fragmented documentation and I feel I provided a better answer in this post here: https://medium.com/@texasdave2/replace-and-remove-a-label-in-a-prometheus-query-9500faa302f0

Replace is not a true REPLACE

Your goal is to simply replace the old label name “old_job_id” with a new label name “new_task_id”. Prometheus label_replace will really “add” the new label name. It will preserve the old label name as well… So, that could be a problem, it’s not a true “replace in place”.

So if you want to “add” your new label name and “remove” the old label name, you need to do this:

sum without (old_job_id) (label_replace(metric, "new_task_id", "$1", "old_job_id", "(.*)"))

Here’s how this reads:

• sum without (old_job_id) will remove the old label name from the query output

• metric is your metric, like “node_filesystem_avail_bytes”

• “new_task_id” is where you would put your new label name

• “$1” is regex for using the string in new label name, don’t change this

• “old_job_id” is where you’ll put your old label, the one you want to get rid of (.*……. that mess is regex that will replace the whole label name

You can have more than one source label, so:

- source_labels: [__name__, name]
  regex: "node_systemd_unit_state;(.+)"
  target_label: unit_name

If the label name isn't matching up with other metrics/exporters, you should file a bug with them. Relabelling like this should only be a temporary solution while a proper fix is being pursued.

Prometheus allows renaming labels in the following places:

• During metric scrape. In this case you need to add the following relabeling rules into metric_relabel_configs section at the corresponding scrape_config:

metric_relabel_configs:
- source_labels: [__name__, name]
  regex: "node_systemd_unit_state;(.+)"
  target_label: unit_name
- source_labels: [__name__, name]
  regex: "node_systemd_unit_state;(.+)"
  target_label: name
  replacement: ""

The first rule copies name label value into unit_name label for metrics with node_systemd_unit_state name. The second rule sets name label value to an empty string (e.g. deletes the name label) for metrics with node_systemd_unit_state name.

• During querying via label_join() function:

label_join(
  label_join(node_systemd_unit_state, "unit_name", "", "name"),
  "name", "", "non_existing_label"
)

The inner label_join() copies name label into unit_name label. The outer label_join() substitutes the original name label with an empty string (e.g. remove the name label).

As you can see, label_join() isn't the best function for label renaming. The label_replace() isn't the best function for label renaming too. While Prometheus doesn't provide better solutions for label renaming, such solution exist in Prometheus-like systems such as VictoriaMetrics (I'm the author of this system). It provides label_move() function:

label_move(node_systemd_unit_state, "name", "unit_name")

Additionally, VictoriaMetrics provides if option for conditional relabeling rules. For example, the following relabeling rules are equivalent to the rules above, but are easier to understand and maintain:

metric_relabel_configs:
- if: 'node_systemd_unit_state{name!=""}'
  source_labels: [name]
  target_label: unit_name
- if: 'node_systemd_unit_state{name!=""}'
  target_label: name
  replacement: ""