Prometheus support

[kereis]

Hey everyone!

I'd like to implement Prometheus support for our project "LibreWB". You can consider it as an alternative to InfluxDB.

If things work out well, I'll create a pull request.

[andig]

Sounds good. Please let me know if you need support! The. Ain task would be mapping the plethora of measurements to Prometheus metric types, the rest should be staightforward?

[kereis]

I guess 👍 . Currently

• I added a router for /metrics in the existing HTTP daemon and a YAML property api.prometheus (maybe move it to prometheus as standalone) that can be set if Prometheus support is needed.
• Added a "Prometheusd" struct which takes care of registring Prometheus metrics and also provides a Run method for snip runners.

I decided to create Prometheusd even though Prometheus Go library provides a global default registry. But I think it does make sense to have a "single instance of responsibility" for Prometheus stuff. Technically it's not really a daemon so it may be renamed at some point.

[kereis]

I'm slowly getting close to finish. I have a "proposal" for refactoring Measurement structs (for a better context, check out commit 19e54e9.):

• I refactored Measurement to be more strongly typed.
• Units are now typed as well with enumer and struct Unit.
• I implemented a optional pattern which allows flexible creation of future Measurement structs, without adding too unnecessary clutter like filling parameters for factory function with empty/nil values.
• Required measurementOptions for creating a Measurement are:
  • WithDescription(string)
  • WithUnit(string)
  • WithMetricType(MetricType)

Moreover, for Measurement specifically, I implemented a dynamic creation of Prometheus metrics.

• At its current state, Prometheus metric names and help text for Measurement are derrived from a Measurement's description and unit.
  1. Measurement's description is transformed to lower case and each whitespace character is replaced with an underscore character
  2. Description and the Measurement's Unit's plural form are concatenated with underscores.
     • If the Unit's plural form is not present, we simply take the alternative representation (for Prometheus) and append an s.
     • The default plural form can be overridden by simply setting a custom plural form for a Unit
  3. In order to distinguish measurement metrics, measurement is prepended.
  4. For help texts:
     • If a Unit is present: Measurement of DESCRIPTION in UNIT
     • Else: Measurement of DESCRIPTION
  5. Possible name at Prometheus metric creation: mbmd_measurement_l1_power_watts
• If you don't like these "auto-generated" names and help texts, you can simply override them by using the measurementOptions (however, make sure to follow the usual Prometheus naming conventions):
  • WithPrometheusName(string)
  • WithPrometheusDescription(string)

The last steps would be to check where metrics can be embedded as well and to test if the result of /metrics HTTP API is good for us. :)