Prometheus label values are not escaped correctly

[tvhong-amazon]

Describe the bug

Hi,
While testing the Prometheus metrics, I noticed that the metric labels are not escaped correctly.

Example ra_metrics for 2 quorum queues:

• "qq on the default vhost
• queue \qq on the vhost "myvhost:

# TYPE rabbitmq_detailed_raft_log_commit_index gauge
# HELP rabbitmq_detailed_raft_log_commit_index Raft log commit index
rabbitmq_detailed_raft_log_commit_index{vhost=""myvhost",queue="\qq"} 2
rabbitmq_detailed_raft_log_commit_index 8
rabbitmq_detailed_raft_log_commit_index{vhost="/",queue=""qq"} 2
rabbitmq_detailed_raft_log_commit_index 10
# TYPE rabbitmq_detailed_raft_log_last_written_index gauge
# HELP rabbitmq_detailed_raft_log_last_written_index Raft log last written index
rabbitmq_detailed_raft_log_last_written_index{vhost=""myvhost",queue="\qq"} 2
rabbitmq_detailed_raft_log_last_written_index 8
rabbitmq_detailed_raft_log_last_written_index{vhost="/",queue=""qq"} 2
rabbitmq_detailed_raft_log_last_written_index 10

Reproduction steps

1. Start a RabbitMQ node locally.
2. Create a quorum queue with name "qq.
3. (Optional) Create a new vhost "myvhost and add a quorum queue named \qq.
4. Access metrics endpoint: http://localhost:15692/metrics/detailed?family=ra_metrics .
5. Observe that the queue labels are not escaped correctly.

Expected behavior

Label values are escaped correctly as per Prometheus Exposition Format.

label_value can be any sequence of UTF-8 characters, but the backslash (\), double-quote ("), and line feed (\n) characters have to be escaped as \\, \", and \n, respectively.

For example:

rabbitmq_detailed_raft_log_commit_index{vhost="\"myvhost",queue="\\qq"} 2
rabbitmq_detailed_raft_log_commit_index{vhost="/",queue="\"qq"} 2

Additional context

I took a quick look at the dependent prometheus.erl library. Looks like it escapes the label values. Not sure why RabbitMQ doesn't.

https://github.com/deadtrickster/prometheus.erl/blob/b78e17eda43c7fcb2c05f76c7dd80db969cd8479/src/formats/prometheus_text_format.erl#L238-L245