Move MonotonicEndTime to only use (#5443)

Have the code definition live next to its usage.

Co-authored-by: Sam Xie <sam@samxie.me>

sdk/trace/span.go

@@ -19,7 +19,6 @@ import (
 	"go.opentelemetry.io/otel/codes"
 	"go.opentelemetry.io/otel/internal/global"
 	"go.opentelemetry.io/otel/sdk/instrumentation"
-	"go.opentelemetry.io/otel/sdk/internal"
 	"go.opentelemetry.io/otel/sdk/resource"
 	semconv "go.opentelemetry.io/otel/semconv/v1.25.0"
 	"go.opentelemetry.io/otel/trace"
@@ -385,7 +384,7 @@ func (s *recordingSpan) End(options ...trace.SpanEndOption) {
 
 	// Store the end time as soon as possible to avoid artificially increasing
 	// the span's duration in case some operation below takes a while.
-	et := internal.MonotonicEndTime(s.startTime)
+	et := monotonicEndTime(s.startTime)
 
 	// Do relative expensive check now that we have an end time and see if we
 	// need to do any more processing.
@@ -436,6 +435,16 @@ func (s *recordingSpan) End(options ...trace.SpanEndOption) {
 	}
 }
 
+// monotonicEndTime returns the end time at present but offset from start,
+// monotonically.
+//
+// The monotonic clock is used in subtractions hence the duration since start
+// added back to start gives end as a monotonic time. See
+// https://golang.org/pkg/time/#hdr-Monotonic_Clocks
+func monotonicEndTime(start time.Time) time.Time {
+	return start.Add(time.Since(start))
+}
+
 // RecordError will record err as a span event for this span. An additional call to
 // SetStatus is required if the Status of the Span should be set to Error, this method
 // does not change the Span status. If this span is not being recorded or err is nil