feat: filter metric value using regex

[raedtwj]

this change let you filter metric values using regex

Ex config.yml

---
modules:
  default:
    metrics:
    - name: available_memory
      path: '{ .availableMemory }'
      help: Available memory in MB
      valuetype: gauge
      regex: ^\d*\.?\d*  # only match digits

this config will be parsed

{"availableMemory": "1000 MB"}

to

# HELP available_memory Available memory in MB
# TYPE available_memory gauge
available_memory 1000

and won't throwing parse error since the suffix is not matched by the pattern

[rustycl0ck]

Please add unit tests and update the documentation/configuration file example so that users can understand how this feature works and how to use it.

The implementation looks good to me. Thanks for the PR!

[rustycl0ck]

Suggested change

	Regex string
	IncludeRegex string

[dansimov04012022]

That feature would be very helpful, cause' it seems like there's no other way to convert string values like this 1.333285ms (duration, 1 millisecond and 333285 nanoseconds) to float 1.333285.

[liubo20210831]

@rustycl0ck
We are looking forward to this feature very much because I want to use JSON Exporter to obtain a value with a percentage sign, such as 66.66%. If this work can be completed as soon as possible, it would be great. We are very grateful!

Thanks for the PR!

[eli-eli333]

We have been using this feature for some time and it is very useful to us. It fully meets the expected requirements. We hope you can merge this request as soon as possible. This is really important for us. Thank you very much !!!!!!

[eli-eli333]

For example, using Gauge metrics to represent memory usage, number of connections, etc. Can convert string to float 1.52GB.

We have been using this feature for some time and it is very useful to us. It fully meets the expected requirements. We hope you can merge this request as soon as possible. This is really important for us. Thank you very much !!!!!!

[ylazaar]

we cannot alter the source json. We cannot apply any transformation on the exporter's side. How are we supposed to extract any non-float data as a metric in order to plot it in grafana. We seriously need this feature or a similar solution.

[ewul]

@SuperQ
Hello, how can we push this PR as soon as possible, since it seems to be very useful for everyone.

[raedtwj]

Please add unit tests and update the documentation/configuration file example so that users can understand how this feature works and how to use it.

The implementation looks good to me. Thanks for the PR!

Documentation and unit tests have been added, Special thanks to @eli-eli333 and @liubo20210831