[doc] Metrics documentation lacks metadata information

[Patrick Petit]

The structure of a StackLight metric is defined in the developer documentation at http://lma-developer-guide.readthedocs.io/en/latest/metrics.html, but this documentation is not up to date.
For example, it misses the 'dimensions' field and the 'hostname' is said mandatory while it is optional. Furthermore, the list of supported metrics at http://lma-developer-guide.readthedocs.io/en/latest/metrics.html fails to document what optional metadata like 'tenant_id' are available for what metric so that users could create their own custom dashboards using metadata tags.