Cancel/Discard Started Performance Custom Traces 🚀

## What feature would you like to see?

Possibility to discard/cancel custom Firebase Performance trace. Currently we can only start and stop trace which always will track time. eg.

```
val myTrace = Firebase.performance.newTrace("test_trace")
myTrace.start()

// code that you want to trace

myTrace.stop()
```
But if for some reason I want to discard started trace
(example scenario: I want to trace how long app fetches data from api but some error occurs and I don't want to track partially downloaded data).
I cannot do it and I have to live with started trace which takes memory on device.

## How would you use it?

I think adding method `Trace#cancel()` which under the hood will unsubscribe from all observations would be best solution.