add WithSpanObserver function

common.go

@@ -12,8 +12,14 @@ import (
 // ErrUnsupported is an error returned when the underlying driver doesn't provide a given function.
 var ErrUnsupported = errors.New("operation unsupported by the underlying driver")
 
-// TagQuery is a span tag for SQL queries.
-const TagQuery = "query"
+// spanTagQuery is a span tag for SQL queries.
+var spanTagQuery = "query"
+
+// SetTagQuery sets a span tag for SQL queries.
+// Use in init funcs only.
+func SetTagQuery(tag string) {
+	spanTagQuery = tag
+}
 
 // SpanNameFunc defines a function which returns a name for the span which is being created on traceable operations.
 // Passing span naming function is optional, however it gives the user a way to use a custom naming strategy. To allow
@@ -23,9 +29,10 @@ type SpanNameFunc func(context.Context) string
 
 // tracer defines a set of instances for collecting spans.
 type tracer struct {
-	t         opentracing.Tracer
-	nameFunc  SpanNameFunc
-	saveQuery bool
+	t            opentracing.Tracer
+	nameFunc     SpanNameFunc
+	observerFunc func(context.Context, opentracing.Span)
+	saveQuery    bool
 }
 
 // newSpan creates a new opentracing.Span instance from the given context.
@@ -37,6 +44,9 @@ func (t *tracer) newSpan(ctx context.Context) opentracing.Span {
 		opts = append(opts, opentracing.ChildOf(parent.Context()))
 	}
 	span := t.t.StartSpan(name, opts...)
+	if t.observerFunc != nil {
+		t.observerFunc(ctx, span)
+	}
 	return span
 }
 

conn.go

@@ -71,7 +71,7 @@ func (c *conn) Exec(query string, args []driver.Value) (driver.Result, error) {
 func (c *conn) ExecContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Result, error) {
 	s := c.tracer.newSpan(ctx)
 	if c.tracer.saveQuery {
-		s.SetTag(TagQuery, query)
+		s.SetTag(spanTagQuery, query)
 	}
 	defer s.Finish()
 	if execerContext, ok := c.conn.(driver.ExecerContext); ok {
@@ -107,7 +107,7 @@ func (c *conn) Query(query string, args []driver.Value) (driver.Rows, error) {
 func (c *conn) QueryContext(ctx context.Context, query string, args []driver.NamedValue) (rows driver.Rows, err error) {
 	s := c.tracer.newSpan(ctx)
 	if c.tracer.saveQuery {
-		s.SetTag(TagQuery, query)
+		s.SetTag(spanTagQuery, query)
 	}
 	defer s.Finish()
 	if queryerContext, ok := c.conn.(driver.QueryerContext); ok {

driver.go

@@ -24,20 +24,6 @@ func NewTracingDriver(d driver.Driver, t opentracing.Tracer, options ...func(*tr
 	return td
 }
 
-// SpanNameFunction is an option for using a custom span naming function.
-func SpanNameFunction(f SpanNameFunc) func(*tracingDriver) {
-	return func(d *tracingDriver) {
-		d.tracer.nameFunc = f
-	}
-}
-
-// SaveQuery is an option for saving SQL queries.
-func SaveQuery(f SpanNameFunc) func(*tracingDriver) {
-	return func(d *tracingDriver) {
-		d.tracer.saveQuery = true
-	}
-}
-
 // Open implements driver.Driver Open.
 func (d *tracingDriver) Open(name string) (driver.Conn, error) {
 	c, err := d.driver.Open(name)

options.go

@@ -0,0 +1,28 @@
+package sql
+
+import (
+	"context"
+
+	"github.com/opentracing/opentracing-go"
+)
+
+// SpanNameFunction is an option for using a custom span naming function.
+func SpanNameFunction(f SpanNameFunc) func(*tracingDriver) {
+	return func(d *tracingDriver) {
+		d.tracer.nameFunc = f
+	}
+}
+
+// SaveQuery is an option for saving SQL queries.
+func SaveQuery(f SpanNameFunc) func(*tracingDriver) {
+	return func(d *tracingDriver) {
+		d.tracer.saveQuery = true
+	}
+}
+
+// WithSpanObserver allows you to modify the span's tags for every span created.
+func WithSpanObserver(obsFunc func(context.Context, opentracing.Span)) func(*tracingDriver) {
+	return func(d *tracingDriver) {
+		d.tracer.observerFunc = obsFunc
+	}
+}

stmt.go

@@ -35,7 +35,7 @@ func (s *stmt) Query(args []driver.Value) (driver.Rows, error) {
 func (s *stmt) ExecContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Result, error) {
 	span := s.tracer.newSpan(ctx)
 	if s.tracer.saveQuery {
-		span.SetTag(TagQuery, query)
+		span.SetTag(spanTagQuery, query)
 	}
 	defer span.Finish()
 	if execerContext, ok := s.stmt.(driver.ExecerContext); ok {
@@ -52,7 +52,7 @@ func (s *stmt) ExecContext(ctx context.Context, query string, args []driver.Name
 func (s *stmt) QueryContext(ctx context.Context, query string, args []driver.NamedValue) (rows driver.Rows, err error) {
 	span := s.tracer.newSpan(ctx)
 	if s.tracer.saveQuery {
-		span.SetTag(TagQuery, query)
+		span.SetTag(spanTagQuery, query)
 	}
 	defer span.Finish()
 	if queryerContext, ok := s.stmt.(driver.QueryerContext); ok {