trace: add starttime start option

Adds a StartTime option when creating a new span.
In some cases, you are able to trace only after the operation
was made. for example in some post-operation hook/observer.

    func myHook(ctx context.Context, info queryInfo) {
       _, span := StartSpan("mydatabase", WithStartTime(time.Now().Sub(info.Duration()))
       span.End()
    }

trace/trace.go

@@ -141,6 +141,10 @@ type StartOptions struct {
 	// SpanKind represents the kind of a span. If none is set,
 	// SpanKindUnspecified is used.
 	SpanKind int
+
+	// StartTime is used as the start time of the span if provided. Else it will
+	// use time.Now().
+	StartTime time.Time
 }
 
 // StartOption apply changes to StartOptions.
@@ -161,6 +165,13 @@ func WithSampler(sampler Sampler) StartOption {
 	}
 }
 
+// WithStartTime sets the span start time.
+func WithStartTime(t time.Time) StartOption {
+	return func(o *StartOptions) {
+		o.StartTime = t
+	}
+}
+
 // StartSpan starts a new child span of the current span in the context. If
 // there is no span in the context, creates a new trace and span.
 //
@@ -236,7 +247,7 @@ func startSpanInternal(name string, hasParent bool, parent SpanContext, remotePa
 
 	span.data = &SpanData{
 		SpanContext:     span.spanContext,
-		StartTime:       time.Now(),
+		StartTime:       o.StartTime,
 		SpanKind:        o.SpanKind,
 		Name:            name,
 		HasRemoteParent: remoteParent,
@@ -246,6 +257,9 @@ func startSpanInternal(name string, hasParent bool, parent SpanContext, remotePa
 	span.messageEvents = newEvictedQueue(cfg.MaxMessageEventsPerSpan)
 	span.links = newEvictedQueue(cfg.MaxLinksPerSpan)
 
+	if span.data.StartTime.IsZero() {
+		span.data.StartTime = time.Now()
+	}
 	if hasParent {
 		span.data.ParentSpanID = parent.SpanID
 	}

trace/trace_test.go

@@ -230,6 +230,20 @@ func TestStartSpanWithRemoteParent(t *testing.T) {
 	}
 }
 
+func TestStartSpanWithStartTime(t *testing.T) {
+	start := time.Now()
+
+	ctx, span := StartSpan(context.Background(), "parent", WithSampler(AlwaysSample()), WithStartTime(start))
+	if !span.data.StartTime.Equal(start) {
+		t.Errorf("expected start time=%s was=%s", start, span.data.StartTime)
+	}
+
+	_, span = StartSpan(ctx, "child")
+	if !span.data.StartTime.After(start) {
+		t.Error("expected child's start time to be after parent's")
+	}
+}
+
 // startSpan returns a context with a new Span that is recording events and will be exported.
 func startSpan(o StartOptions) *Span {
 	_, span := StartSpanWithRemoteParent(context.Background(), "span0",