There are two problems,

1. We have a read permission issue of /var/log/nginx/gs_cache.access.log:
chromeos-test@chromeos-gt-devserver17:/var/log/upstart$ ls -l /var/log/nginx/gs-cache.access.log
-rw-r----- 1 www-data adm 1422 Sep 25 12:14 /var/log/nginx/gs-cache.access.log

The metric script is run as 'chromeos-test', so we cannot read it.

2. The tail script raises broken pipe error every time when new content comes to the file:

ts_mon was set up.
Attempting refresh to obtain initial access_token
Refreshing access_token
Finished waiting for ts_mon process.
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/local/google/home/chromeos-test/chromiumos/chromeos-admin/venv/file_utils/tail_until_writer_finished.py", line 98, in <module>
    main()
  File "/usr/local/google/home/chromeos-test/chromiumos/chromeos-admin/venv/file_utils/tail_until_writer_finished.py", line 94, in main
    seek_to_end=not args.from_beginning)
  File "/usr/local/google/home/chromeos-test/chromiumos/chromeos-admin/venv/file_utils/tail_until_writer_finished.py", line 69, in TailFile
    ReadChunks(fh)
  File "/usr/local/google/home/chromeos-test/chromiumos/chromeos-admin/venv/file_utils/tail_until_writer_finished.py", line 62, in ReadChunks
    outfile.flush()
IOError: [Errno 32] Broken pipe
Tue Sep 25 12:14:58 PDT 2018 gs_cache_nginx_metrics: exited.