Normalize new syntax from recent PRs

[Mr0grog]

This is a quick pass at updating some of the syntax to match newer language features I used in recent PRs (#69, #71):

• Array .includes(x) instead of .indexOf(x) !== -1.
• for...of loops.
• Rest/spread (...) syntax, and using that to modify arrays (where it’s safe!) instead of calling concat(), which can be slower and more memory-intense.
• Don’t use callbackify where not necessary.
• Arrow functions (where they do something useful).

• Newer style for functions in object literals:

  
  const object = {
    someFunction (arg1, arg2) {
      // ...function body here...
    }
  };
  // is equivalent to:
  const object = {
    someFunction: function someFunction (arg1, arg2) {
      // ...function body here...
    }
  };

This does not cover larger syntactical/structural changes, like:

• class syntax.
• Promises instead of callbacks.
• async/await keywords.

Those are probably better suited to a separate PR, but I can also do them here if we think it makes more sense.

[Mr0grog]

Don't merge! Found a bug, adding a test to cover it.

[Mr0grog]

Fixed — I modified all the metrics to include a copy of their tags on the flushed results, so an aggregator or reporter that edits one tag list can’t accidentally change other flushed metrics at the same time.