Metrics generator regex policy filter compiles on each attribute value match

[andriusluk]

Is your feature request related to a problem? Please describe.
Metrics generator has a filter policy with match type regex. Generator is iterating over attributes and performs regex compilation every time it tries to match value. This results in high memory consumption.
https://github.com/grafana/tempo/blob/main/pkg/spanfilter/spanfilter.go#L85

Describe the solution you'd like
Compile regex once and perform match with already compiled regex.

Describe alternatives you've considered
Used strict match type on the policy, which works well and memory consumption is stable. Although I have a use-case where I would like to use regex, but can't afford the memory consumption.

Additional context
Using strict type used to consume just over 1.2GB of memory. Changing to config presented below resulted in ~60GB of memory being used.

filter_policies:
  - include:
      match_type: regex
      attributes:
        - key: resource.telemetry.sdk.version
          value: 1\.1[6-9]\.[0-9]
        - key: service.namespace
          value: payments|access

[joe-elliott]

Nice find. This would be a nice improvement, but I don't have time to get to it atm. Going to tag it to look for some help.