chore(issues): add metric to inferring project platform (#95803)

Follow up to #95402, adding a metric to see how often this path gets
hit. We don't expect it to be used much, so sample rate = 1.

Author: cvxluo

src/sentry/event_manager.py

@@ -718,6 +718,14 @@ def _set_project_platform_if_needed(project: Project, event: Event) -> None:
                 event=audit_log.get_event_id("PROJECT_EDIT"),
                 data={**project.get_audit_log_data(), "platform": event.platform},
             )
+            metrics.incr(
+                "issues.infer_project_platform.success",
+                sample_rate=1.0,
+                tags={
+                    "reason": "new_project" if not project.first_event else "backfill",
+                    "platform": event.platform,
+                },
+            )
 
     except Exception:
         logger.exception("Failed to infer and set project platform")