Add User-Agent header and documentation (awslabs#34)

* Add User-Agent documentation to README

* Add User-Agent header value

---------

Co-authored-by: Trevor Bonas <[email protected]>

README.md

@@ -768,6 +768,14 @@ remote_read:
 The Prometheus Connector will use the default credentials provider implemented in the AWS SDK for Go instead of allowing users 
 to provide the credentials through command-line flags. This prevents sensitive data from being easily scraped.
 
+## User-Agent Header
+
+The Prometheus Connector uses the following `User-Agent` header for all requests:
+
+```
+User-Agent: Prometheus Connector/<version> aws-sdk-go/<version> (go<version>; <os>; <cpu arch>)
+```
+
 # Developer Documentation
 
 ## Building the Prometheus Connector from Source

timestream/client.go

@@ -22,6 +22,7 @@ import (
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/aws/credentials"
+    "github.com/aws/aws-sdk-go/aws/request"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/timestreamquery"
 	"github.com/aws/aws-sdk-go/service/timestreamquery/timestreamqueryiface"
@@ -41,19 +42,26 @@ import (
 type labelOperation string
 type longMetricsOperation func(measureValueName string) (labelOperation, error)
 
+var addUserAgent = request.NamedHandler {
+    Name: "UserAgentHandler",
+    Fn: request.MakeAddToUserAgentHandler("Prometheus Connector", Version),
+}
+
 // Store the initialization function calls to allow unit tests to mock the creation of real clients.
 var initWriteClient = func(config *aws.Config) (timestreamwriteiface.TimestreamWriteAPI, error) {
 	sess, err := session.NewSession(config)
 	if err != nil {
 		return nil, err
 	}
+    sess.Handlers.Build.PushFrontNamed(addUserAgent)
 	return timestreamwrite.New(sess), nil
 }
 var initQueryClient = func(config *aws.Config) (timestreamqueryiface.TimestreamQueryAPI, error) {
 	sess, err := session.NewSession(config)
 	if err != nil {
 		return nil, err
 	}
+    sess.Handlers.Build.PushFrontNamed(addUserAgent)
 	return timestreamquery.New(sess), nil
 }