support postgres for gorm v2 (#833)

* support postgres for gorm v2 and adding docs

docs/instrumenting.asciidoc

@@ -411,6 +411,30 @@ func main() {
 }
 ----
 
+==== module/apmgormv2
+Package apmgormv2 provides a means of instrumenting http://gorm.io[GORM] database operations.
+
+To trace `GORM` operations, import the appropriate `apmgormv2/driver` package (instead of the
+`gorm.io/driver` package), use these dialects to `gorm.Open` instead of gorm drivers.
+
+Once you have a `*gorm.DB`, you can call `db.WithContext` to
+propagate a context containing a transaction to the operations:
+
+[source,go]
+----
+import (
+	"gorm.io/gorm"
+	postgres "go.elastic.co/apm/module/apmgormv2/driver/postgres"
+)
+
+func main() {
+	db, err := gorm.Open(postgres.Open("dsn"), &gorm.Config{})
+	...
+	db = db.WithContext(ctx)
+	db.Find(...) // creates a "SELECT FROM <foo>" span
+}
+----
+
 [[builtin-modules-apmgocql]]
 ==== module/apmgocql
 Package apmgocql provides a means of instrumenting https://github.com/gocql/gocql[gocql] so

docs/supported-tech.asciidoc

@@ -141,7 +141,10 @@ operation.
 As with `database/sql` support we provide additional support for the
 postgres, mysql, and sqlite dialects.
 
-See <<builtin-modules-apmgorm, module/apmgorm>> for more information
+We provide different packages for the Gorm v1 and v2 versions:
+`module/apmgorm` for Gorm v1.x, and `module/apmgormv2` for Gorm v2.x.
+
+See <<builtin-modules-apmgorm, module/apmgorm>> or <<builtin-modules-apmgorm, module/apmgormv2>> for more information
 about GORM instrumentation.
 
 [float]

module/apmgormv2/apmgorm_test.go

@@ -33,6 +33,7 @@ import (
 
 	"go.elastic.co/apm/apmtest"
 	mysql "go.elastic.co/apm/module/apmgormv2/driver/mysql"
+	postgres "go.elastic.co/apm/module/apmgormv2/driver/postgres"
 	sqlite "go.elastic.co/apm/module/apmgormv2/driver/sqlite"
 	"go.elastic.co/apm/module/apmsql"
 )
@@ -51,6 +52,22 @@ func TestWithContext(t *testing.T) {
 		)
 	})
 
+	if pgHost := os.Getenv("PGHOST"); pgHost == "" {
+		t.Logf("PGHOST not specified, skipping")
+	} else {
+		t.Run("postgres", func(t *testing.T) {
+			testWithContext(t,
+				apmsql.DSNInfo{
+					Address:  pgHost,
+					Port:     5432,
+					Database: "test_db",
+					User:     "postgres",
+				},
+				postgres.Open("user=postgres password=hunter2 dbname=test_db sslmode=disable"), &gorm.Config{},
+			)
+		})
+	}
+
 	if mysqlHost := os.Getenv("MYSQL_HOST"); mysqlHost == "" {
 		t.Logf("MYSQL_HOST not specified, skipping")
 	} else {

module/apmgormv2/driver/postgres/init.go

@@ -0,0 +1,41 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+// +build go1.14
+
+// Package apmpostgres imports the gorm mysql dialect package,
+// and also registers the mysql driver with apmsql.
+package apmpostgres
+
+import (
+	"gorm.io/driver/postgres"
+	"gorm.io/gorm"
+
+	apmpgxv4 "go.elastic.co/apm/module/apmsql/pgxv4"
+)
+
+// Open creates a dialect with apmsql
+func Open(dsn string) gorm.Dialector {
+	dialect := &postgres.Dialector{
+		Config: &postgres.Config{
+			DriverName: apmpgxv4.DriverName,
+			DSN:        dsn,
+		},
+	}
+
+	return dialect
+}

module/apmgormv2/go.mod

@@ -4,8 +4,8 @@ require (
 	github.com/stretchr/testify v1.5.1
 	go.elastic.co/apm v1.8.0
 	go.elastic.co/apm/module/apmsql v1.8.0
-	golang.org/x/sys v0.0.0-20200223170610-d5e6a3e2c0ae // indirect
 	gorm.io/driver/mysql v1.0.2
+	gorm.io/driver/postgres v1.0.2
 	gorm.io/driver/sqlite v1.1.4-0.20200928065301-698e250a3b0d
 	gorm.io/gorm v1.20.2
 )