Add labels to sysctls related to clocks.

Sysctls like kern.eventtimer.et.*.quality currently embed the name of
the clock device. This is problematic for the Prometheus metrics
exporter for two reasons:

- Some of those clocks have dashes in their names, which Prometheus
  doesn't allow to be used in metric names.
- It doesn't allow for extracting the same property of all clocks on the
  system from within a single query.

Attach these nodes to have a label, so that the Prometheus metrics
exporter gives these metric a uniform name with the name of the clock
attached as a label.

Reviewed by:	cem
Differential Revision:	https://reviews.freebsd.org/D8775

sys/kern/kern_et.c

@@ -66,9 +66,9 @@ et_register(struct eventtimer *et)
 		}
 	}
 	KASSERT(et->et_start, ("et_register: timer has no start function"));
-	et->et_sysctl = SYSCTL_ADD_NODE(NULL,
+	et->et_sysctl = SYSCTL_ADD_NODE_WITH_LABEL(NULL,
 	    SYSCTL_STATIC_CHILDREN(_kern_eventtimer_et), OID_AUTO, et->et_name,
-	    CTLFLAG_RW, 0, "event timer description");
+	    CTLFLAG_RW, 0, "event timer description", "eventtimer");
 	SYSCTL_ADD_INT(NULL, SYSCTL_CHILDREN(et->et_sysctl), OID_AUTO,
 	    "flags", CTLFLAG_RD, &(et->et_flags), 0,
 	    "Event timer capabilities");

sys/kern/kern_tc.c

@@ -1218,9 +1218,9 @@ tc_init(struct timecounter *tc)
 	/*
 	 * Set up sysctl tree for this counter.
 	 */
-	tc_root = SYSCTL_ADD_NODE(NULL,
+	tc_root = SYSCTL_ADD_NODE_WITH_LABEL(NULL,
 	    SYSCTL_STATIC_CHILDREN(_kern_timecounter_tc), OID_AUTO, tc->tc_name,
-	    CTLFLAG_RW, 0, "timecounter description");
+	    CTLFLAG_RW, 0, "timecounter description", "timecounter");
 	SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(tc_root), OID_AUTO,
 	    "mask", CTLFLAG_RD, &(tc->tc_counter_mask), 0,
 	    "mask for implemented bits");