Transform metrics use handles

[rukai]

Currently the chain registers metrics for all its transforms but then throws away the handles, requiring the transforms to access the metrics without the handles.
This currently works fine however there are two issues with this:

• its a lot slower to access metrics without the handle, it was showing up in a profiler.
• Its easy to forget to register the metric in the first place when working without handles. In fact thats exactly what had happened to the transform_pushed_* metrics.

This PR fixes those two issues by holding onto the handles and then using them later on for recording metrics.

After the change I can now see:

• that we are spending less time in metrics code in the windsock samply profiler
• A roughly 25% improvement in our transform/loopback micro benchmark introduced by add loopback microbenchmark #1402 (I had to alter it slightly to enable metrics otherwise the difference wont show up)