…eap profiler, try 2

Adding the "skipped samples" placeholder breaks the heap profiler, as
it starts failing with:

> WARN -- datadog: [datadog] CpuAndWallTimeWorker thread error. Cause:
> RuntimeError Ended a heap recording that was not started
> WARN -- datadog: [datadog] Detected issue with profiler (worker
> component), stopping profiling.

This is because the stack recorder up until now used the check for
`alloc_samples > 0` as a proxy for "this is an allocation sample, and
someone already called `track_object(...)` with it".

The addition of the skipped samples placeholder broke this assumption,
as it uses `alloc_samples > 0` but explicitly there's no object being
tracked.

To fix this issue, I've added a new argument to the `sample_values`
that explicitly marks a sample as also being heap (or not). Thus,
we can now explicitly tell the stack recorder which situation we are in
using that flag, instead of trying to derive it from `alloc_samples >
0`.

This is actually the second attempt at fixing this. The first attempt
got reverted along with
#3775 because it tried to
detect placeholders (and thus used a placeholder as a proxy for
"someone called `track_object(...)`". This was incorrect because
there's other placeholders than just the "skipped samples" (case in
point: the "In native code" placeholder), which can happen together
with an object allocation.