perf expr: Accumulate rather than replace in the context counts

perf expr: Accumulate rather than replace in the context counts

Metrics will fill in the context to have mappings from an event to a
count. When counts are added they replace existing mappings which
generally shouldn't exist with aggregation. Switch to accumulating to
better support cases where perf stat's aggregation isn't used and we
may see a counter more than once.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250710235126.1086011-8-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>