perf metricgroups: Add NO_THRESHOLD_AND_NMI constraint

perf metricgroups: Add NO_THRESHOLD_AND_NMI constraint

Thresholds can increase the number of counters a metric needs. The NMI
watchdog can take away a counter (hopefully the buddy watchdog will
become the default and this will no longer be true). Add a new
constraint for the case that a metric and its thresholds would fit in
counters but only if the NMI watchdog isn't enabled. Either the
threshold or the NMI watchdog should be disabled to make the metric
fit. Wire this up into the metric__group_events logic.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250719030517.1990983-16-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>