clean up and document prometheus-nginxlog-exporter

[HarHarLinks]

Describe the bug

• There are 2 nginx-proxy.json grafana dashboards in 2 places.
• It is not clear from the docs how to make use of the metrics, e.g. prefixes etc.
• There are typos in the config e.g.

  matrix-docker-ansible-deploy/roles/custom/matrix-prometheus-nginxlog-exporter/templates/prometheus-nginxlog-exporter.yaml.j2

  Line 9 in e3cbc61

  preffix: "myprefix"

• overall the config seems wrong, it deploys with settings such as namespace_label: "namespace" and a format that does not appear to fit the nginx logs
• it tries to read from syslog even though the matrix_synapse_reverse_proxy_companion_access_log_syslog_integration_enabled is false - which however would appear to fix the log format
• ...