Generate docs on available metrics from metrics.yaml file and improve it

[andy-maier]

The current documentation on available metrics is a separate copy in the usage.rst file. As a separate copy, there is the risk of discrepancies between the actual metric definition and that documentation.

This ticket is to generate the metrics documentation from the metric definition file.

On that opportunity, some deficiencies of the current metrics documentation should be addressed. The generated metrics documentation should:

• Group the metrics by metric group, because that is the granularity at which metrics can be enabled or disabled.
• Provide the attributes shown in the current documentation (exported metric name, CPC modes, metric type, metric description)
• Provide the labels of each metric (label name and description)
• Provide the origin of the metrics (HMC metric name / resource type & property name)
• Provide information about HMC and SE/CPC version dependencies of each metric

This requires some additional data to be defined in the metric definition file:

• Add a description to the labels
• Add the CPC modes (on the metric group?)
• Add human readable version information to the metrics, where needed (they already exist as comments in the metric definition file)