standardize commonly used attributes

Some commonly used at maal attributes have been encoded as consts with
convinience wrappers similar to those of semconv package from otel sdk.
Additionally some utils that can generate mutliple attributes were added.

example/main.go

@@ -70,7 +70,7 @@ func main() {
 }
 
 func Serv(ctx context.Context) *fiber.Error {
-	ctx, span := tracer.Service(ctx, "name of the span")
+	ctx, span := tracer.Service(ctx, "service", "service span")
 	defer span.End()
 
 	for range []int{1, 2, 3} {
@@ -86,7 +86,7 @@ func Serv(ctx context.Context) *fiber.Error {
 }
 
 func Repo(ctx context.Context) error {
-	ctx, span := tracer.Repository(ctx, "name of the span")
+	ctx, span := tracer.Repository(ctx, "repo", "repo span")
 	defer span.End()
 
 	for range []int{1, 2, 3} {