Require explicit ExecKind in SpawnMetrics.Builder and remove SpawnMetrics.EMPTY

Previously we used `REMOTE` as the default but now we'll require setting
`ExecKind` explicitly. To make it less error prone, I've introduced a few
factory methods that set it. Also, as previously discussed, I've introduced
a new `ExecKind.OTHER` (IMHO sounds a bit better than the initial proposal
of `INTERNAL`, but I don't feel strongly, so let me know if you prefer
the latter).

We also discussed not setting `ExecKind` in `SpawnMetrics.EMPTY` (and using
something like `null`), but I found it a bit tricky to really define what
that would actually mean, now that we have `ExecKind` (e.g., should we allow
aggregating such "empty" values? if yes, should it be commutative? etc.)
But I've noticed that after the introduction of `AggregatedSpawnMetrics`,
we only use `EMPTY` in one place and it's possible to simply remove it, which
avoids the problem.

RELNOTES: None.
PiperOrigin-RevId: 314687611