New conversion function for extracted labels

[ash211]

Is your feature request related to a problem? Please describe.
I have a log line with a field in ISO 8601 duration format (wikipedia). Example: PT26.481S. I'd like to parse this into a number of milliseconds for use in charting.

https://grafana.com/docs/loki/latest/query/metric_queries/#unwrapped-range-aggregations documents two parsing functions related to this:

• duration_seconds (and synonym duration)
• bytes

Neither of these supports this format.

Describe the solution you'd like
I'd like to propose another conversion function:

• duration_millis_iso8601

This function takes as input a String in ISO 8601 duration format, and returns a numeric value representing the number of milliseconds in the duration. This is the same string->f64 signature as the other two functions.

Describe alternatives you've considered
I considered supplying my own conversion function, but from the above documentation about metric queries it doesn't seem that this plugin point is supported.

Thanks for consideration

[ash211]

Possibly related: #2925