perf: optimize SQL based database instrumentation (#955)

* perf: optimize SQL-based database instrumentation (#931, #938)

Author: Angith

context.go

@@ -22,9 +22,5 @@ func ContextWithSpan(ctx context.Context, sp ot.Span) context.Context {
 // span, this method returns false.
 func SpanFromContext(ctx context.Context) (ot.Span, bool) {
 	sp, ok := ctx.Value(activeSpanKey).(ot.Span)
-	if !ok {
-		return nil, false
-	}
-
-	return sp, true
+	return sp, ok
 }

context_test.go

@@ -26,6 +26,7 @@ func TestSpanFromContext_WithActiveSpan(t *testing.T) {
 }
 
 func TestSpanFromContext_NoActiveSpan(t *testing.T) {
-	_, ok := instana.SpanFromContext(context.Background())
+	sp, ok := instana.SpanFromContext(context.Background())
+	assert.Equal(t, sp, nil)
 	assert.False(t, ok)
 }