Ability to override transaction_sample_rate on a per controller basis

[jclusso]

Is your feature request related to a problem? Please describe.

Sampling is not very usefull when you have a few controllers that make up most of your applications traffic.

Describe the solution you'd like

Ability to override the transaction_sample_rate on a per controller basis.

Describe alternatives you've considered

Playing around with ElasticAPM.add_filter, but not sure this is ideal.