easy enablement for prometheus related feature

[NOTE]: # ( ^^ Provide a general summary of the request in the title above. ^^ )

## Summary
Easy enablement for prometheus feature by environment variables

[NOTE]: # ( Provide a brief overview of what the new feature is all about. )

## Desired Behaviour

[NOTE]: # ( Tell us how the new feature should work. Be specific. )
[TIP]: # ( Do NOT give us access or passwords to your actual CouchDB! )
For sample as `COUCHDB_USER`, as a user, I want to enable prometheus related feature by environment value as
```
[prometheus]
additional_port = false
bind_address = 127.0.0.1
port = 17986
```

`prometheus_additional_port`
`prometheus_bind_address`
`prometheus_port`

## Possible Solution

[NOTE]: # ( Not required. Suggest how to implement the addition or change. )
Not sure if https://github.com/apache/couchdb/blob/main/configure#L247 this line is useless.
if not, take prometheus_port as sample:
```
{prometheus_port, $prometheus_port}
```
and in
rel/overlay/etc/default.ini
```
[prometheus]
additional_port = false
bind_address = 127.0.0.1
port = {{prometheus_port}}
```

## Additional context

[TIP]: # ( Why does this feature matter to you? What unique circumstances do you have? )
N/A

Start by reading the Prometheus-related configuration in rel/overlay/etc/default.ini and the prometheus_port entry near configure#L247. Trace how existing environment variables reach these settings and confirm the behavior for prometheus_additional_port, prometheus_bind_address, and prometheus_port. Done means these environment values can enable and configure the Prometheus feature consistently.

Written by the indexing model from the issue text.