pkg/profiler: Change metric description

While this does fix the double registration panic issue,
further work needs to be done to expose relevant labels.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>

pkg/profiler/profiler.go

@@ -100,8 +100,9 @@ func NewCgroupProfiler(
 		mtx: &sync.RWMutex{},
 		missingStacks: promauto.With(reg).NewCounterVec(
 			prometheus.CounterOpts{
-				Name: "parca_agent_profiler_missing_stacks_total",
-				Help: "Number of missing profile stacks",
+				Name:        "parca_agent_profiler_missing_stacks_total",
+				Help:        "Number of missing profile stacks",
+				ConstLabels: map[string]string{"target": target.String()},
 			},
 			[]string{"type"}),
 	}