allow Histograms and Summaries to time Callables (2)

[yarosla]

@brian-brazil Recreated #368 from squashed branch + signed-off

Another usability enhancement allowing instead of:

String v;
try (Summary.Timer timer = summary.startTimer()) {
  v = someMethod();
}

to write:

String v = summary.time(this::someMethod);

This is especially useful when programming (reactive) streams.

Trying to make metric code as unobtrusive as possible.

BTW Micrometer already does that.

[brian-brazil]

Thanks!