Fix BaseBucketApi.add incrementing _total per call instead of per value

[Chessing234]

Bug

`BaseBucketApi.total`'s docstring is "Return the total number of values added to the tracker", but `BaseBucketApi.add` does:

```python
for value, count in zip(values, counts):
self._add(value, count)
self._total += 1
self._sum += value * count
```

So `tracker.add(values=[v0, v1], counts=[5, 3])` ends up with `_total == 2` instead of `8`, while `_sum` is correctly weighted by `count`.

Root cause

The increment is per loop iteration rather than per observation. This is exactly the discrepancy that `add_summary` works around:

```python
def add_summary(self, summary: SummaryTuple):
prev_count, prev_sum = self._total, self._sum
self.add(summary.bins, summary.counts)
# override the total and sum with the previous values, thats' because otherwise they are approximate
self._total = prev_count + summary.total
self._sum = prev_sum + summary.sum
```

Without the override, the total after rebuilding a tracker via `SummarySpec.to_tracker(...)` (which calls `tracker.add(values=self.bins, counts=self.counts)`) reflects only the number of bins, not the original observation count.

Fix

Increment `_total` by `count`, matching `_sum`'s per-value accumulation and the documented contract. The `add_summary` override remains correct (it now agrees with what `add` produces).