feat: set 'static_value' per metric config

[burningalchemist]

Description

There are situations when we want to return some string data from the query (to re-process it later on the Prometheus side), and there's no value columns that could represent float64 number as a metric value. Even though we can solve it with some SQL adjustments, it's not always clear, when we look at SQL statements alone. It's also possible, that we simply cannot add a additional dummy value column due to different constraints (e.g. stored procedures we can't change) to point values parameter to.

To address this problem, we introduce metric configuration field static_value which can be set to a fixed number. The result is that we focus on the information mapped to key_labels instead, the value itself doesn't need to mean anything (although, the commonly used value would be 1) and can be ignored later on.

values and static_value are mutually exclusive parameters.

References

• https://www.robustperception.io/numbers-from-displaystrings-with-the-snmp_exporter/
• https://www.robustperception.io/converting-string-states-to-booleans-with-the-jmx-exporter/