add support to monitor specified process's mem,cpu

sometimes,we do not care all the process or system's memory,cpu , but just the ones which we are interest in. the ProcessMetrics support this.
so does the FileMetrics 
This test doesn't do anything. Please delete it.

Also, please add tests for FileMetrics and ProcessMetrics. 
Writing tests can be tricky when using Sigar, but for example you could have this test for FileMetrics:

1. Create a temporary file with a known size
2. Create a FileMetrics for that file.
3. Ask the FileMetrics for the size of the file, and check that it matches the real file size.