feat: add custom labels to sloth_slo_info metric

[cxdy]

Adds the ability to define info_labels to be appended to the sloth_slo_info{} metric

For example, we can generate this template:

version: "prometheus/v1"
service: "nagios"
labels:
  component: "nagios"
  environment: "production"
# this file is used to generate prometheus rules with sloth - https://github.com/slok/sloth
# to use it, run:
# sloth generate -i sloth/nagios.yml -o infra/nagios_slo.yml
# uses default window of 30 days
# window defined during generation https://sloth.dev/usage/slo-period-windows/#custom-slo-period-catalog
slos:
  - name: "nagios-availability"
    objective: 90 # low due to 5 min window and scrape_interval of 30s
    description: "Nagios Availability via prometheus-nagios-exporter scrapes"
    info_labels:
      infoLabelOne: true
      infoLabelTwo: "hello world"
    sli:
      events: # https://stackoverflow.com/a/71846686
        error_query: sum(count_over_time(up{job="nagios_exporter",environment="production"}[{{.window}}]) - sum_over_time(nagios_up{environment="production", job="nagios_exporter" ,component="nagios"}[{{.window}}]))
        total_query: sum(count_over_time(up{job="nagios_exporter",environment="production"}[{{.window}}]))
    .....

Into this:

---
# Code generated by Sloth (dev): https://github.com/slok/sloth.
# DO NOT EDIT.

groups:
- name: sloth-slo-sli-recordings-nagios-nagios-availability
  rules:
  ....
  - record: sloth_slo_info
    expr: vector(1)
    labels:
      component: nagios
      environment: production
      infoLabelOne: true
      sloth_id: nagios-nagios-availability
      sloth_mode: cli-gen-prom
      sloth_objective: "90"
      sloth_service: nagios
      sloth_slo: nagios-availability
      sloth_spec: prometheus/v1
      sloth_version: dev
      infoLabelTwo: "hello world"
  ....

Redacted a bunch of output for readability but you get the gist.

[dennisme]

looks good!