Fix config token config detection (#2495)

* Fix config token config detection

* Add unit tests

internal/pkg/agent/configuration/fleet_server.go

@@ -69,7 +69,7 @@ func ElasticsearchFromConnStr(conn string, serviceToken, serviceTokenPath string
 			VerificationMode: tlscommon.VerifyNone,
 		}
 	}
-	if serviceToken == "" {
+	if serviceToken == "" && serviceTokenPath == "" {
 		return Elasticsearch{}, errors.New("invalid connection string: must include a service token")
 	}
 	cfg.ServiceToken = serviceToken

internal/pkg/agent/configuration/fleet_server_test.go

@@ -0,0 +1,69 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package configuration
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/elastic/elastic-agent/internal/pkg/agent/errors"
+)
+
+func TestElasticsearchFromConnStr(t *testing.T) {
+	testcases := []struct {
+		name     string
+		conn     string
+		token    string
+		path     string
+		insecure bool
+		es       Elasticsearch
+		err      error
+	}{{
+		name:     "ok",
+		conn:     "https://localhost:9200",
+		token:    "my-token",
+		path:     "",
+		insecure: false,
+		es: Elasticsearch{
+			Protocol:     "https",
+			Hosts:        []string{"localhost:9200"},
+			ServiceToken: "my-token",
+		},
+		err: nil,
+	}, {
+		name:     "ok with path",
+		conn:     "https://localhost:9200",
+		token:    "",
+		path:     "/path/to/token",
+		insecure: false,
+		es: Elasticsearch{
+			Protocol:         "https",
+			Hosts:            []string{"localhost:9200"},
+			ServiceTokenPath: "/path/to/token",
+		},
+		err: nil,
+	}, {
+		name:     "no token or path",
+		conn:     "https://localhost:9200",
+		token:    "",
+		path:     "",
+		insecure: false,
+		es:       Elasticsearch{},
+		err:      errors.New("invalid connection string: must include a service token"),
+	}}
+
+	for _, tc := range testcases {
+		t.Run(tc.name, func(t *testing.T) {
+			es, err := ElasticsearchFromConnStr(tc.conn, tc.token, tc.path, tc.insecure)
+			if tc.err != nil {
+				assert.EqualError(t, err, tc.err.Error())
+			} else {
+				assert.NoError(t, err)
+			}
+			assert.Equal(t, tc.es, es)
+		})
+	}
+}