Ignoremetrics: add feature to ignore Values returned by modbus device

[bastischubert]

Devices like CMS700 from ABB send Error codes as Values which break monotonicity for counters.

Example:
Watt Hours should only be increasing in a normal rate. CMS 700 sends out 4.294.934.512 Wh if the sensor has not yet returned data, which can result in sudden GWh consumtion within the scrape interval if you use a "increase()" on that metric.

[RichiH]

That will make the sensor disappear from the scrape though, correct?

Wouldn't it be better to force the value to be 0 in this case?

[bastischubert]

That will raise an error which is handled "upwards" and only return an error for that scrape:

./modbus_exporter  -config.file modbus_ignored.yml 
INFO[0000] Loading configuration file modbus_ignored.yml  source="modbus_exporter.go:49"
INFO[0000] telemetry metrics at: :9602                   source="modbus_exporter.go:56"
INFO[0000] Modbus metrics at: :9602                      source="modbus_exporter.go:64"
INFO[0003] got scrape request for module 'F454-L3-watthours' target '172.26.4.11:8001' and sub_target '2'  source="modbus_exporter.go:98"
ERRO[0003] failed to scrape metrics for module 'F454-L3-watthours': metric 'watt_hours', address '38992': ignored value returned as value: value is 1 - TestValue  source="modbus_exporter.go:113"

Returning a 0 value instead would break the monotonicity for counters (eg. you have 4000 kwh consumed, drop to 0 for one errornous scrape and than be on 4001 on the next scrape. increase() would return "increased by 4001" instead of just one)