add gc_count metrics with correct collection unit .

fixes #3549

Author: david-gang

instrumentation/opentelemetry-instrumentation-system-metrics/src/opentelemetry/instrumentation/system_metrics/__init__.py

@@ -469,6 +469,12 @@ def _instrument(self, **kwargs: Any):
                     description=f"Runtime {self._python_implementation} GC count",
                     unit="By",
                 )
+                self._meter.create_observable_counter(
+                    name=f"process.runtime.{self._python_implementation}.gc_count",
+                    callbacks=[self._get_runtime_gc_count],
+                    description="The number of times a generation was collected since interpreter start.",
+                    unit="{collection}",
+                )
 
         if "process.runtime.thread_count" in self._config:
             self._meter.create_observable_up_down_counter(