supervisor: add support for including local configuration files (open-telemetry#38671)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

This PR enables the supervisor to read the local configuration files as
per the spec, and merge those with other sources to be included in the
final merged effective config.

The local configuration files are read last, and as such, any values
there take precedence over other sources.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes open-telemetry#37886

<!--Describe what testing was performed and which tests were added.-->
#### Testing

Amended the existing test for composing configuration to include two
extra local files, and made sure the end-result is merged properly on
effective_config.yaml

<!--Please delete paragraphs that you did not use before submitting.-->

---------

Signed-off-by: Paschalis T <[email protected]>
Signed-off-by: Paschalis Tsilias <[email protected]>

Author: tpaschalis

.chloggen/tpaschalis-opampsupervisor-merge-local-config.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: opampsupervisor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: add support for including local configuration files
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37886]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

cmd/opampsupervisor/supervisor/supervisor.go

@@ -38,6 +38,7 @@ import (
 	"go.opentelemetry.io/collector/config/configopaque"
 	"go.opentelemetry.io/collector/config/configtelemetry"
 	"go.opentelemetry.io/collector/config/configtls"
+	"go.opentelemetry.io/collector/confmap"
 	"go.opentelemetry.io/collector/pdata/pcommon"
 	semconv "go.opentelemetry.io/collector/semconv/v1.21.0"
 	"go.opentelemetry.io/contrib/bridges/otelzap"
@@ -1035,6 +1036,36 @@ func (s *Supervisor) composeOpAMPExtensionConfig() []byte {
 	return cfg.Bytes()
 }
 
+func (s *Supervisor) composeAgentConfigFiles() []byte {
+	conf := confmap.New()
+
+	for _, file := range s.config.Agent.ConfigFiles {
+		cfgBytes, err := os.ReadFile(file)
+		if err != nil {
+			s.telemetrySettings.Logger.Error("Could not read local config file", zap.Error(err))
+			continue
+		}
+
+		cfgMap, err := yaml.Parser().Unmarshal(cfgBytes)
+		if err != nil {
+			s.telemetrySettings.Logger.Error("Could not unmarshal local config file", zap.Error(err))
+			continue
+		}
+		err = conf.Merge(confmap.NewFromStringMap(cfgMap))
+		if err != nil {
+			s.telemetrySettings.Logger.Error("Could not merge local config file: "+file, zap.Error(err))
+			continue
+		}
+	}
+
+	b, err := yaml.Parser().Marshal(conf.ToStringMap())
+	if err != nil {
+		s.telemetrySettings.Logger.Error("Could not marshal merged local config files", zap.Error(err))
+		return []byte("")
+	}
+	return b
+}
+
 func (s *Supervisor) loadAndWriteInitialMergedConfig() error {
 	var lastRecvRemoteConfig, lastRecvOwnTelemetryConfig []byte
 	var err error
@@ -1228,6 +1259,10 @@ func (s *Supervisor) composeMergedConfig(config *protobufs.AgentRemoteConfig) (c
 		return false, err
 	}
 
+	if err = k.Load(rawbytes.Provider(s.composeAgentConfigFiles()), yaml.Parser(), koanf.WithMergeFunc(configMergeFunc)); err != nil {
+		return false, err
+	}
+
 	// The merged final result is our new merged config.
 	newMergedConfigBytes, err := k.Marshal(yaml.Parser())
 	if err != nil {

cmd/opampsupervisor/supervisor/supervisor_test.go

@@ -176,7 +176,7 @@ func Test_composeEffectiveConfig(t *testing.T) {
 	s := Supervisor{
 		telemetrySettings:            newNopTelemetrySettings(),
 		persistentState:              &persistentState{},
-		config:                       config.Supervisor{Capabilities: config.Capabilities{AcceptsRemoteConfig: acceptsRemoteConfig}},
+		config:                       config.Supervisor{Capabilities: config.Capabilities{AcceptsRemoteConfig: acceptsRemoteConfig}, Agent: config.Agent{ConfigFiles: []string{"testdata/local_config1.yaml", "testdata/local_config2.yaml"}}},
 		pidProvider:                  staticPIDProvider(1234),
 		hasNewConfig:                 make(chan struct{}, 1),
 		agentConfigOwnMetricsSection: &atomic.Value{},

cmd/opampsupervisor/supervisor/testdata/local_config1.yaml

@@ -0,0 +1,6 @@
+receivers:
+  journald:
+    units:
+      - ssh
+    priority: info
+    directory: /run/log/journal

cmd/opampsupervisor/supervisor/testdata/local_config2.yaml

@@ -0,0 +1,3 @@
+exporters:
+  file/2:
+    path: ./foo

cmd/opampsupervisor/testdata/collector/effective_config.yaml

@@ -1,6 +1,8 @@
 exporters:
     file:
         path: /test/logs/output.log
+    file/2:
+        path: ./foo
 extensions:
     health_check:
         endpoint: localhost:8000
@@ -20,6 +22,11 @@ receivers:
         include:
             - /test/logs/input.log
         start_at: beginning
+    journald:
+        directory: /run/log/journal
+        priority: info
+        units:
+            - ssh
 service:
     extensions:
         - health_check