Add note about instance relabeling for grafana dashboards

[robbi5]

First, thanks for the awesome bbb-exporter 😍

After setting it up a few minutes ago, I was a bit confused about missing cpu/bandwidth-metrics. The instance dropdown in grafana is build by using label_values(bbb_api_up, instance) - in my case that returned bbbhost.example:443.
The instance label from my netdata appeared as bbbhost.example:19999. In this case, the $instance variable in grafana contained bbbhost.example:443 and such the corresponding netdata metrics were not found.

Currently I've solved it by adding the following relabel config to my scrape_configs in prometheus.yml:

relabel_configs:
- source_labels: ['__address__']
  separator:     ':'
  regex:         '(.*):.*'
  target_label:  'instance'
  replacement:   '$1'

Just FYI - if you like, just close the issue - or add a note about the requirement of equal instance labels for netdata and the exporter for the dashboards :)

[greenstatic]

Thank you @robbi5 for your findings. Yeah, I made the assumption that both the exporter and Netdata will be behind the BigBlueButton Nginx reverse proxy. I will expand the documentation to include this. Thanks!