The filter processor should drop the metrics if all metrics are filtered out.

[jjzeng-seattle]

Describe the bug
Per title.

Steps to reproduce
Setting up a pipeline with filter processor and stackdriver exporter, and config filter to filter out all metrics, you will see the error on stackdriver exporter non-nil metric or metric descriptor

What did you expect to see?
The metrics should be drop and not reach the exporters, if it's empty.

What did you see instead?
A clear and concise description of what you saw instead.

What version did you use?
Version: (e.g., v0.4.0, 1eb551b, etc)

What config did you use?
Config: (e.g. the yaml config file)

Environment
OS: (e.g., "Ubuntu 20.04")
Compiler(if manually compiled): (e.g., "go 14.2")

Additional context
Add any other context about the problem here.

[jjzeng-seattle]

/assign @anniefu