Use lsof to monitor fds on MacOS

[domdom82]

• A short explanation of the proposed change:
  This is just a small fix to be able to run gorouter under MacOS for development purposes.
  The PR changes the fd_monitor so that it distinguishes between Linux (which has /proc) and MacOS (which has not).

As a result, this annoying error message will be gone:

{"log_level":3,"timestamp":1645613724.0060532,"message":"error-reading-filedescriptor-path","source":"gorouter.stdout.FileDescriptor","data":{"error":"open /proc/<pid>/fd: no such file or directory"}}
(repeats every 5s)

Instead of outright removing the fd_monitor functionality, I've opted to make it work under MacOS by shelling out to lsof to count the open file descriptors. Since gorouter will be deployed to Linux in production scenarios, this may be overkill but I wanted to retain the original functionality and not just turn it off. However, I can also just put in a no-op to turn off the error message instead.

• An explanation of the use cases your change solves
  It allows to develop gorouter under MacOS without breaking the fd_monitor

• Instructions to functionally test the behavior change using operator interfaces (BOSH manifest, logs, curl, and metrics)

1. go build on MacOS
2. nats-server &
3. ./gorouter

• Expected result after the change
  Gorouter starts up, connects to natsd and shows no error messages

• Current result before the change
  Gorouter starts up, connects to natsd and keeps logging the above error message every 5s

• Links to any other associated PRs

• I have viewed signed and have submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using scripts/run-unit-tests-in-docker from routing-release.

• (Optional) I have run Routing Acceptance Tests and Routing Smoke Tests on bosh lite

• (Optional) I have run CF Acceptance Tests on bosh lite

[ameowlia]

LGTM! Apologizes for the wait.

[ameowlia]

Bumped in routing-release: cloudfoundry/routing-release@b8c7078