perf stat: Add aggr_nr metric parser support

perf stat: Add aggr_nr metric parser support

Introduce the `aggr_nr` function to the metric expression parser.
`aggr_nr` allows metric formulas to dynamically utilize the number of
aggregated targets (`aggr->nr`) instead of relying on the static
`source_count` (which represents the static socket or node count).

This adds the `AGGR_NR` token to the lexer and parser, updates the
expression parsing context helpers to store `aggr_nr`, and feeds
`aggr->nr` from the aggregation structure in `prepare_metric`.

Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
Tested-by: Zide Chen <zide.chen@intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Thomas Falcon <thomas.falcon@intel.com>
Cc: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>