Tmp validation fix for self instrumentation cfg

Validation `nonzero` is broken with latest beats update. Temporarily
implement validation instead of using validation tags until
elastic/go-ucfg#147 is fixed.

beater/config/instrumentation.go

@@ -21,6 +21,7 @@ import (
 	"time"
 
 	"github.com/elastic/beats/libbeat/logp"
+	"github.com/elastic/go-ucfg"
 )
 
 const (
@@ -33,12 +34,21 @@ const (
 type InstrumentationConfig struct {
 	Enabled     *bool           `config:"enabled"`
 	Environment *string         `config:"environment"`
-	Hosts       urls            `config:"hosts" validate:"nonzero"`
+	Hosts       urls            `config:"hosts"` //TODO(simi): add `validate:"nonzero"` again once https://github.com/elastic/go-ucfg/issues/147 is fixed
 	Profiling   ProfilingConfig `config:"profiling"`
 	APIKey      string          `config:"api_key"`
 	SecretToken string          `config:"secret_token"`
 }
 
+func (c *InstrumentationConfig) Validate() error {
+	for _, h := range c.Hosts {
+		if h == nil || h.Host == "" {
+			return ucfg.ErrZeroValue
+		}
+	}
+	return nil
+}
+
 // IsEnabled indicates whether self instrumentation is enabled
 func (c *InstrumentationConfig) IsEnabled() bool {
 	// self instrumentation is disabled by default.

beater/config/instrumentation_test.go

@@ -0,0 +1,47 @@
+// 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.
+
+package config
+
+import (
+	"testing"
+
+	"github.com/elastic/go-ucfg"
+
+	"github.com/elastic/beats/libbeat/common"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNonzeroHosts(t *testing.T) {
+	t.Run("ZeroHost", func(t *testing.T) {
+		cfg, err := NewConfig("9.9.9",
+			common.MustNewConfigFrom(map[string]interface{}{
+				"instrumentation.enabled": true, "instrumentation.hosts": []string{""}}), nil)
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), ucfg.ErrZeroValue.Error())
+		assert.Nil(t, cfg)
+	})
+
+	t.Run("Valid", func(t *testing.T) {
+		cfg, err := NewConfig("9.9.9",
+			common.MustNewConfigFrom(map[string]interface{}{"instrumentation.enabled": true}), nil)
+		require.NoError(t, err)
+		assert.True(t, *cfg.SelfInstrumentation.Enabled)
+		assert.Empty(t, cfg.SelfInstrumentation.Hosts)
+	})
+}