Split cpu time metric into user/system components like prof.log

The total can be calculated from the two parts via Prometheus/Grafana
if desired, so it's more informative to pass them as separate parts.

src/telemetry/ProcessStats.h

@@ -9,7 +9,8 @@ namespace zeek::telemetry::detail {
 struct process_stats {
     int64_t rss = 0;
     int64_t vms = 0;
-    double cpu = 0.0;
+    double cpu_user = 0.0;
+    double cpu_system = 0.0;
     int64_t fds = 0;
 };