Only report gc_time metric if profiler is enabled

[tombruijn]

Let's only report the gc_time metric when there's an actual value to
report to avoid confusion of very small GC times.

If the Profiler is disabled, it reports 0.0 as the value for this
metric. We know when the profiler is enabled by asking it
GC::Profiler.enabled?.

By directly asking the Ruby GC profiler, instead of our wrapper based on
the enable_gc_instrumentation config option, we allow users to enable
and disable the profiler temporarily with GC::Profiler.enable and
GC::Profiler.disable calls in their app.

If the profiler is enabled, disabled, and then enabled again, it
remembers the total time from the last time it was enabled. By not
reporting 0 when it is disabled we don't reset the cache and don't
trigger a very high duration being reported the next time it's enabled.

> GC::Profiler.total_time
# => 0.0

> GC::Profiler.enable
> GC::Profiler.total_time
=> 0.001
> 10000.times { SecureRandom.uuid }
# => Does stuff for a long time
> GC::Profiler.total_time
# => 58.93548299997067

> GC::Profiler.disable
> GC::Profiler.total_time
# => 0.0
> 10000.times { SecureRandom.uuid }
# => Does stuff for a long time
> GC::Profiler.total_time
# => 0.0
> GC::Profiler.enable
> GC::Profiler.total_time
# => 58.93548299997067
# Same as before it was disabled without the GC time of the code that
# was run when it was disabled

Part of #868