`QueueManager.externalLabels` becomes a slice rather than a `Labels` so
we can index into it when doing the merge operation.
Note we avoid calling `Labels.Len()` in `labelProtosToLabels()`.
It isn't necessary - `append()` will enlarge the buffer and we're
expecting to re-use it many times.
Also, we now validate protobuf input before converting to Labels.
This way we can detect errors first, and we don't place unnecessary
requirements on the Labels structure.
Re-do seriesFilter using labels.Builder (albeit N^2).
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
abd9909595