Benchmarks will allow for easier identification of slow collectors.
Additionally, they increase test coverage of the collectors, with some
collectors now reaching 80-95% coverage with this change.
Collector benchmarks have been structed so that common functionality is
present in `collector/collector_test.go` as is done with non-test
functionality in `collector/collector.go`.
Test logic that is specific to individual collectors is present in the
collector test file (E.G. `collector/process_test.go` for the Process
collector).
Signed-off-by: Ben Reedy <breed808@breed808.com>
