contrib/database/sql: Fix sample code in document to prevent panic on OpenDB

[tsbkw]

What does this PR do?

Fix document to prevent to misuse Register function.
Issue: #1634

Motivation

In the example below, driver.Driver is passed as struct, and reflect.TypeOf(driver) is added to map.

dd-trace-go/contrib/database/sql/sql.go

Lines 12 to 13 in 2d25957

	// sqltrace.Register("pq", pq.Driver{})
	// db, err := sqltrace.Open("pq", "postgres://pqgotest:password@localhost...")

Later when user call OpenDB, reflect.TypeOf(driver) is looked up in d.keys, therefore driver added and looked up must have same type. For example, if added type is struct, looked up by struct. And if added type is pointer of struct, looked up by pointer of struct.

dd-trace-go/contrib/database/sql/sql.go

Line 76 in 2d25957

name, ok := d.keys[reflect.TypeOf(driver)]

This behavior is not a problem when driver doesn't implement driver.DriverContext interface, because in this case driver get from map driver is used when lookup and type of these always same.

dd-trace-go/contrib/database/sql/sql.go

Line 236 in 2d25957

return OpenDB(&dsnConnector{dsn: dataSourceName, driver: d}, opts...), nil

However when driver implement driver.DriverContext, user must care whether type is pointer or not when calling Register.

Driver in lib/pq currently not implement driver.DriverContext but soon it will implement. see: lib/pq#900

Ideally the way look up should be better, but at least document should be fixed.

Describe how to test/QA your changes

The problem can be reproduced by code below.

	sqltrace.Register("mysql", mysql.MySQLDriver{})
	_, err := sqltrace.Open("mysql", "user:password@tcp(localhost:3306)/db")
	fmt.Printf("err with mysql.MySQLDriver{} %v\n", err)

This print panic: sqltrace.OpenDB: driver is not registered via sqltrace.Register with stacktrace.

The error can be avoided by passing by pointer like below.

	sqltrace.Register("mysql", &mysql.MySQLDriver{})
	_, err2 := sqltrace.Open("mysql", "user:password@tcp(localhost:3306)/dbe")
	fmt.Printf("err with &mysql.MySQLDriver{} %v\n", err2)

This print err with &mysql.MySQLDriver{} <nil>.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.