Add row count and affected rows to Trilogy spans

Identifying queries that return/update many rows is very useful for
improving database performance. Queries that return many rows can cause
applications to use excess memory (or in some cases like Vitess) could
be blocked altogether. Queries that update many rows can cause
replication lag and generally put more pressure on a database.

This commit adds both of these values (row_count and affected_rows) to
spans created from Trilogy#query to help identify these problematic
queries. Trilogy::Result#count will always return a value (0 for
mutations), but #affected_rows only returns values for mutations (nil
for SELECTs).

instrumentation/trilogy/lib/opentelemetry/instrumentation/trilogy/patches/client.rb

@@ -49,7 +49,7 @@ def query(sql)
                 OpenTelemetry::Instrumentation::Trilogy.attributes
               ),
               kind: :client
-            ) do |_span, context|
+            ) do |span, context|
               if propagator && sql.frozen?
                 sql = +sql
                 propagator.inject(sql, context: context)
@@ -58,7 +58,14 @@ def query(sql)
                 propagator.inject(sql, context: context)
               end
 
-              super
+              result = super
+
+              attributes = { 'db.response.returned_rows' => result.count }
+              attributes['db.response.affected_rows'] = result.affected_rows unless result.affected_rows.nil?
+
+              span.add_attributes(attributes)
+
+              result
             end
           end
 

instrumentation/trilogy/test/opentelemetry/instrumentation/trilogy/instrumentation_test.rb

@@ -164,6 +164,26 @@
         _(span.attributes[OpenTelemetry::SemanticConventions::Trace::DB_SYSTEM]).must_equal 'mysql'
         _(span.attributes[OpenTelemetry::SemanticConventions::Trace::DB_STATEMENT]).must_equal 'DESELECT ?'
       end
+
+      it 'includes row count' do
+        client.query('SELECT 1')
+
+        _(span.attributes['db.response.returned_rows']).must_equal(1)
+      end
+
+      it 'includes affected rows' do
+        client.query(<<~SQL)
+          CREATE TABLE products (
+            product_id int
+          )
+        SQL
+
+        client.query('INSERT INTO products (product_id) VALUES (1), (2), (3)')
+
+        _(exporter.finished_spans.last.attributes['db.response.affected_rows']).must_equal(3)
+      ensure
+        client.query('DROP TABLE IF EXISTS products')
+      end
     end
 
     describe 'when connecting' do