[collectd 6] Generate better names and labels for legacy metrics.

[octo]

Legacy metrics, i.e. metrics reported with the value_list_t data structure, need to be upgraded to metric_family_t.

The new code puts all metrics into the collectd.v5 namespace. The metric family name is:

• collectd.v5.${plugin}.utilization for metrics with type "percent".
• collectd.v5.${plugin}.io for directional metrics ending in _octets.
• collectd.v5.${type}.${data_source} for metrics with more than one data source (except directional metrics).
• collectd.v5.${type} for metrics with one data source.

If the hostname field is set, it is translated into the host.name resource attribute. Otherwise resource attributes are omitted to trigger the default behavior (like an empty hostname field does).

The labels depend on whether plugin instance, type instance, or both are set:

• Both: ${plugin}=${plugin_instance}, type=${type_instance}
• Plugin instance: ${plugin}=${plugin_instance}
• Type instance: ${plugin}=${type_instance}
• Neither: plugin=${plugin}

This does a decent job of mapping metrics to the new schema, but I do wonder if we should simply put those fields into labels 1-to-1: collectd.v5.plugin=${plugin}, collectd.v5.plugin_instance=${plugin_instance}, collectd.v5.type_instance=${type_instance}. What do you think?

Directional metrics are metrics with two data sources called "read" and "write", or "rx" and "tx". Despite having more than one data source, the data source name is added to a direction label instead of the metric family name.

ChangeLog: collectd: The schema with which metrics are translated from v5 to v6 has been improved.

[eero-t]

Code looks OK. Have just small optimization suggestion(s).

[eero-t]

This does a decent job of mapping metrics to the new schema, but I do wonder if we should simply put those fields into labels 1-to-1: collectd.v5.plugin=${plugin}, collectd.v5.plugin_instance=${plugin_instance}, collectd.v5.type_instance=${type_instance}. What do you think?

No need for hierarchical labels, metric name being such is enough.

[eero-t]

This breaks Prometheus integration. Dot is invalid in metric and label names. Double colon is also invalid for labels, but could be used as separator for metric names. See: https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels

EDIT: that issue was worked around in Prometheus plugin.