Add option to map measurement names to field name into prometheus input

[pyry]

Added support for re-mapping output field names to use measurement name e.g. instead of gauge = 1 output kube_deployment_status_replicas_available = 1. Need for this rose in our project where we are pulling data from kube-state-metrics prometheus client using telegraf and sending it to InfluxDB. We wanted all the metrics to be stored under single measurement as a separate series to enable various operations such calculating ratios such as mean(kube_deployment_status_replicas) / mean(kube_deployment_status_replicas_available) etc. After this PR one would achieve this by setting measurement_name_as_field_name=true and name_override=kube.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

This is something that we have been talking about internally a bit, I think we also need to use the same measurement name in order to use operators and functions:

cpu_usage_user{cpu="cpu3"} 42
cpu_usage_guest{cpu="cpu3"} 42

prometheus,cpu=cpu3 cpu_usage_user=42,cpu_usage_guest=42

One question that has come up is, what to name the measurement?

[pyry]

Yeah, using the option in the pr really makes sense when combined with name_override forcing the measurement to be the same for all metrics from the same input. Configurable measurement name with sensible default such as ´prometheus´ would probably work if one would want to go that direction that this setting should also mutate the measurement name: E.g. if measurement_name_override etc. is set then that is used as a measurement name and field names become the original prometheus metric_name.

[gangseok514]

@danielnelson this PR has no plan to commit?

[gangseok514]

+1

[danielnelson]

I think we won't merge this pr as is, or at least without doing more research, but let's associate it with #4415. I would like someone to take an inventory of our prometheus metric conversions and then we can discuss how to improve the situation.

[sjwang90]

Closing as this was address in #5767.

Feel free to re-open if it's not the case.