tracing: use timeutil for Now() and Since()

Previously, for performance reasons (as well as a few others) we used
`time.Now()` and `time.Since` in the tracing library. But the work in
f782e45 alleviated those concerns, so
we can now switch to using `timeutil` library in two places.
```
Tracer_StartSpanCtx/opts=none-24                                    508ns ± 0%     512ns ± 0%  +0.80%  (p=0.000 n=10+10)
Tracer_StartSpanCtx/opts=real-24                                    510ns ± 1%     515ns ± 1%  +0.96%  (p=0.000 n=10+10)
Tracer_StartSpanCtx/opts=real,logtag-24                             539ns ± 1%     544ns ± 1%  +0.90%  (p=0.000 n=10+10)
Tracer_StartSpanCtx/opts=real,autoparent-24                         480ns ± 0%     486ns ± 1%  +1.26%  (p=0.000 n=10+10)
Tracer_StartSpanCtx/opts=real,manualparent-24                       523ns ± 1%     527ns ± 1%  +0.91%  (p=0.004 n=10+10)
Tracer_StartSpanCtx/opts=real,autoparent,withEventListener-24       755ns ± 0%     765ns ± 0%  +1.40%  (p=0.000 n=10+10)
Tracer_StartSpanCtx/opts=real,manualparent,withEventListener-24     547ns ± 1%     552ns ± 1%  +0.89%  (p=0.000 n=10+10)
Span_GetRecording/root-only-24                                     7.36ns ± 0%    6.75ns ± 0%  -8.26%  (p=0.000 n=10+10)
Span_GetRecording/child-only-24                                    7.36ns ± 0%    6.75ns ± 0%  -8.30%  (p=0.000 n=10+10)
Span_GetRecording/root-child-24                                    7.36ns ± 0%    6.74ns ± 0%  -8.32%  (p=0.000 n=10+10)
RecordingWithStructuredEvent/with-event-listener-24                3.47µs ± 1%    3.47µs ± 1%    ~     (p=0.541 n=10+10)
RecordingWithStructuredEvent/without-event-listener-24             3.31µs ± 0%    3.32µs ± 1%    ~     (p=0.271 n=10+10)
SpanCreation/detached-child=false-24                               1.07µs ± 1%    1.07µs ± 1%    ~     (p=0.955 n=10+10)
SpanCreation/detached-child=true-24                                1.84µs ± 1%    1.82µs ± 1%  -1.30%  (p=0.000 n=10+10)
```
And no changes in allocations.

Epic: None

Release note: None

pkg/testutils/lint/lint_test.go

@@ -731,9 +731,6 @@ func TestLint(t *testing.T) {
 			":!**/embedded.go",
 			":!util/timeutil/time.go",
 			":!util/timeutil/zoneinfo.go",
-			":!util/tracing/span.go",
-			":!util/tracing/crdbspan.go",
-			":!util/tracing/tracer.go",
 			":!cmd/roachtest/tests/gorm_helpers.go",
 		)
 		if err != nil {

pkg/util/tracing/crdbspan.go

@@ -854,7 +854,7 @@ func (s *crdbSpan) getRecordingNoChildrenLocked(
 		// -1 indicates an unfinished Span. For a recording it's better to put some
 		// duration in it, otherwise tools get confused. For example, we export
 		// recordings to Jaeger, and spans with a zero duration don't look nice.
-		rs.Duration = time.Since(rs.StartTime)
+		rs.Duration = timeutil.Since(rs.StartTime)
 		rs.Finished = false
 	} else {
 		rs.Finished = true

pkg/util/tracing/tracer.go

@@ -1130,7 +1130,7 @@ child operation: %s, tracer created at:
 		opts.LogTags = opts.Parent.i.crdb.logTags
 	}
 
-	startTime := time.Now()
+	startTime := timeutil.Now()
 
 	// First, create any external spans that we may need (OpenTelemetry, net/trace).
 	// We do this early so that they are available when we construct the main Span,