Fix double initialisation when two appmetrics instances are started (singleton bug)

[sjanuary]

An issue was discovered with the implementation of the appmetrics global singleton. Essentially a lot of initialization code was being run twice including attaching the probes and the socket.io probe doesn't have a global check for whether it is already attached. This PR only runs the initialization code once.

See RuntimeTools/appmetrics-dash#124

[codecov-io]

Codecov Report

Merging #508 into master will decrease coverage by 0.03%.
The diff coverage is 73.71%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage    60.8%   60.77%   -0.04%     
==========================================
  Files          49       49              
  Lines        3215     3215              
==========================================
- Hits         1955     1954       -1     
- Misses       1260     1261       +1

Impacted Files	Coverage Δ
index.js	75% <73.71%> (ø)	⬆️
headless_zip.js	73.33% <0%> (-2.23%)	⬇️

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[a-roberts]

Confirmed this fixes a problem seen here, thanks @sjanuary for the quick fix!

[gibfahn]

The ignore whitespace diff is much cleaner btw (for reviewers): https://github.com/RuntimeTools/appmetrics/pull/508/files?w=1

You could do an early return;, but I think the if/else is clearer.

[hhellyer]

Looks good and makes sense, we don't want to do any initialisation if appmetrics has already been required.
Node is single threaded so there's no synchronisation issues to worry about.