add flag to remove uptime and n_users

[ehlerst]

This PR allows setting of flags to turn them off for those who wish.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

#5398
#5283
#3837
more...

Flags set to true

system,host=some-clever-name load1=2.18,load5=3.31,load15=3.45,n_cpus=8i 1555475800000000000

Flags set to false (still the default)

system,host=some-clever-name uptime_format=" 0:34" 1555475750000000000
system,host=some-clever-name uptime=2095i 1555475750000000000
system,host=some-clever-name load1=2.36,load5=3.5,load15=3.52,n_cpus=8i,n_users=1i 1555475750000000000

[glinton]

Let's make this a filter. I think just an exclude filter is sufficient. Allowed values would be uptime and users.

filter.NewIncludeExcludeFilter([]string{}, SomethingExclude)

The docker input plugin uses filters and may be a good reference.

[glinton]

Remove this blank line.

[danielnelson]

I feel like this is too many options for such a small plugin.

The only remaining issue with utmp that I know of is when it doesn't exist. I think it would be a good change to skip the error logging if os.IsNotExist(err) and add a note to the README that it requires this file.

With uptime, I haven't seen any isses at all, if its just something you don't want you can add it to the fielddrop option:

[[inputs.system]]
  fielddrop = ["uptime*"]

[ehlerst]

I like @danielnelson idea better, #5742 to replace this.