log_to_metrics counters should support value_field

[Lusitaniae]

Is your feature request related to a problem? Please describe.
I'm currently using fluentd prometheus plugins to generate metrics, one of the metrics is a transmited bytes counter/histogram

Fluentd supports specifying a key which will increment the counter with its value

https://github.com/fluent/fluent-plugin-prometheus?tab=readme-ov-file#counter-type

But fluentbit only seems to use value_field for guages/histograms, and counters always append by 1

Relevant code:

https://github.com/fluent/fluent-bit/blob/master/plugins/filter_log_to_metrics/log_to_metrics.c#L915

Describe the solution you'd like
Make value_field work for counters

Describe alternatives you've considered
Keep using fluentd

Additional context
Trying to move fluentd -> fluentbit