[Heartbeat] Set HTTP user-agent to 'elastic_heartbeat'

By default heartbeat uses the golang user agent, which is blacklisted
by a number of services, including elastic.co. URLs like
https://www.elastic.co/products/beats/heartbeat do not work with the
default go user agent due to such blacklists.

This changes the default user agent to be 'elastic_heartbeat'.

While the HTTP spec allows UAs to add version numbers to the end this PR
does not add the version number for two reasons:

1. Adding the version wouldn't practically be of use to anyone as one UA
version numbers for an uptime check aren't practically useful.
2. It would needlessly add complexity to this commit.

Resolves elastic#10170 (comment)

CHANGELOG.next.asciidoc

@@ -45,6 +45,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Removed the `add_host_metadata` and `add_cloud_metadata` processors from the default config. These don't fit well with ECS for Heartbeat and were rarely used.
 - Fixed/altered redirect behavior. `max_redirects` now defaults to 0 (no redirects). Following redirects now works across hosts, but some timing fields will not be reported. {pull}14125[14125]
 - Removed `host.name` field that should never have been included. Heartbeat uses `observer.*` fields instead. {pull}14140[14140]
+- Changed default user-agent to be `elastic_heartbeat` as the current default `Go-http-client/1.1` is often blacklisted. {pull}14291[14291]
 
 *Journalbeat*
 

heartbeat/monitors/active/http/task.go

@@ -198,6 +198,9 @@ func buildRequest(addr string, config *Config, enc contentEncoder) (*http.Reques
 
 		request.Header.Add(k, v)
 	}
+	if ua := request.Header.Get("User-Agent"); ua == "" {
+		request.Header.Set("User-Agent", "elastic_heartbeat")
+	}
 
 	if enc != nil {
 		enc.AddHeaders(&request.Header)

heartbeat/monitors/active/http/task_test.go

@@ -24,6 +24,8 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -169,3 +171,29 @@ func TestRequestBuildingWithCustomHost(t *testing.T) {
 		assert.Equal(t, "custom-host", request.Header.Get("Host"))
 	}
 }
+
+func TestRequestBuildingWithNoUserAgent(t *testing.T) {
+	request, err := buildRequest("localhost", &Config{}, nilEncoder{})
+
+	require.Nil(t, err)
+	assert.Equal(t, "elastic_heartbeat", request.Header.Get("User-Agent"))
+}
+
+func TestRequestBuildingWithExplicitUserAgent(t *testing.T) {
+	expectedUserAgent := "some-user-agent"
+
+	var config = Config{
+		Check: checkConfig{
+			Request: requestParameters{
+				SendHeaders: map[string]string{
+					"User-Agent": expectedUserAgent,
+				},
+			},
+		},
+	}
+
+	request, err := buildRequest("localhost", &config, nilEncoder{})
+
+	require.Nil(t, err)
+	assert.Equal(t, expectedUserAgent, request.Header.Get("User-Agent"))
+}