Add the label for using multiple servers

[vmotov]

Is your feature request related to a problem? Please describe

When I use multiple MongoDB servers, such as test and prod, I can't differentiate them in metrics
Let say, I defined servers below

servers:
- name: test
  uri: 'mongodb://readuser:readpassword@hostname_test:27017'
- name: prod
  uri: 'mongodb://readuser:readpassword@hostname_prod:27017'

And next I want to use both servers for one metric

servers: [] #Can also be empty, if empty the metric will be used for every server defined

But in the end, on the /metrics page, I see only one metric.

mongodb_query_exporter_query_total{aggregation="aggregation_4",result="SUCCESS",server="prod"} 1
mongodb_query_exporter_query_total{aggregation="aggregation_4",result="SUCCESS",server="test"} 1
sample_metric_name_gauge{mylabel="mylabel"} 1

And I would like to see something like this

mongodb_query_exporter_query_total{aggregation="aggregation_4",result="SUCCESS",server="prod"} 1
mongodb_query_exporter_query_total{aggregation="aggregation_4",result="SUCCESS",server="test"} 1
sample_metric_name_gauge{server="test", mylabel="mylabel"} 1
sample_metric_name_gauge{server="prod", mylabel="mylabel"} 3

Describe the solution you'd like

Is it possible to add a label with the name of the server to make difference between them?
Please let me know if such a possibility already exists. I didn't find this in Readme

[raffis]

Legit request, this should be doable 👍🏻