Add metric for results of pid lookup in procstat input

[danielnelson]

Feature Request

Proposal:

There is interest in being able to more easily monitor if a process is running as part of the procstat input (#2687). However, mixing the results of the pid lookup with the metrics about the process itself doesn't mesh well with matching multiple pids.

Current behavior:

If no process is matched Telegraf logs:

E! Error: procstat getting process, exe: [foobar] pidfile: [] pattern: [] user: [] Error running /usr/bin/pgrep: exit status 1

No metrics are emitted.

Desired behavior:

procstat_lookup,pid_finder=pgrep,pattern=foobar pid_count=0i

The log message would be removed.

Use case: [Why is this important (helps with prioritizing requests)]

See #2687

[danielnelson]

Closed in #4307