Removing experiment and treamtent labels from experiment engine metrics

[leonlnj]

This PR removes the experiment and treatments for experiment engine metrics.

The flexibility to allow experiment engine to log additional labels which might cause a huge cardinality.

Also, it is recommended not use dimensions with high cardinality -
https://prometheus.io/docs/practices/naming/
https://www.robustperception.io/cardinality-is-key

CAUTION: Remember that every unique combination of key-value label pairs represents a new time series, 
which can dramatically increase the amount of data stored.
Do not use labels to store dimensions with high cardinality (many different label values), 
such as user IDs, email addresses, or other unbounded sets of values.