Consume APMConfig from input units in agent mode (#3277)

* Consume APMConfig from input units in agent mode

Consume and prefer APMConfig from the expected input component while
operating in agent mode over config in inputs[0].server.instrumentation.

---------

Co-authored-by: Julia Bardi <90178898+juliaElastic@users.noreply.github.com>

changelog/fragments/1707852625-Prefer-elastic-agent-client-APMConfig-in-agent-mode.yaml

@@ -0,0 +1,36 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: enhancement
+
+# Change summary; a 80ish characters long description of the change.
+summary: Prefer elastic-agent-client APMConfig in agent mode
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+description: |
+  While running in agent-mode fleet-server will use the APMConfig
+  settings of expected input if it's set over the settings in
+  inputs[0].server.instrumentation; this should make it easier for
+  managing agents to inject APM config data.
+
+# Affected component; a word indicating the component this changeset affects.
+component:
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+pr: 3277
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+issue: 2868

internal/pkg/server/agent.go

@@ -20,6 +20,7 @@ import (
 	"github.com/rs/zerolog"
 
 	"github.com/elastic/elastic-agent-client/v7/pkg/client"
+	"github.com/elastic/elastic-agent-client/v7/pkg/proto"
 	"github.com/elastic/go-ucfg"
 	"gopkg.in/yaml.v3"
 )
@@ -285,7 +286,7 @@ func (a *Agent) start(ctx context.Context) error {
 		return a.reconfigure(ctx)
 	}
 
-	cfg, err := a.configFromUnits()
+	cfg, err := a.configFromUnits(ctx)
 	if err != nil {
 		return err
 	}
@@ -331,7 +332,7 @@ func (a *Agent) reconfigure(ctx context.Context) error {
 		return a.start(ctx)
 	}
 
-	cfg, err := a.configFromUnits()
+	cfg, err := a.configFromUnits(ctx)
 	if err != nil {
 		return err
 	}
@@ -373,7 +374,7 @@ func (a *Agent) stop() {
 
 // configFromUnits takes both inputUnit and outputUnit and creates a single configuration just like fleet server was
 // being started from a configuration file.
-func (a *Agent) configFromUnits() (*config.Config, error) {
+func (a *Agent) configFromUnits(ctx context.Context) (*config.Config, error) {
 	agentID := ""
 	agentVersion := ""
 	agentInfo := a.agent.AgentInfo()
@@ -420,10 +421,54 @@ func (a *Agent) configFromUnits() (*config.Config, error) {
 		return nil, err
 	}
 
+	if expAPMCFG := expInput.APMConfig; expAPMCFG != nil {
+		instrumentationCfg, err := apmConfigToInstrumentation(expAPMCFG)
+		if err != nil {
+			zerolog.Ctx(ctx).Warn().Err(err).Msg("Unable to parse expected APM config as instrumentation config")
+		} else {
+			obj := map[string]interface{}{
+				"inputs": []interface{}{map[string]interface{}{
+					"server": map[string]interface{}{
+						"instrumentation": instrumentationCfg,
+					},
+				},
+				}}
+			err = cfgData.Merge(obj, config.DefaultOptions...)
+			if err != nil {
+				zerolog.Ctx(ctx).Warn().Err(err).Msg("Failed to merge APM config into cfgData")
+			}
+		}
+
+	}
+
 	cliCfg := ucfg.MustNewFrom(a.cliCfg, config.DefaultOptions...)
 	err = cliCfg.Merge(cfgData, config.DefaultOptions...)
 	if err != nil {
 		return nil, err
 	}
 	return config.FromConfig(cliCfg)
 }
+
+// apmConfigToInstrumentation transforms the passed APMConfig into the Instrumentation config that is used by fleet-server.
+func apmConfigToInstrumentation(src *proto.APMConfig) (config.Instrumentation, error) {
+	if apmest := src.GetElastic(); apmest != nil {
+		apmTLS := apmest.GetTls()
+		iTLS := config.InstrumentationTLS{
+			SkipVerify:        apmTLS.GetSkipVerify(),
+			ServerCertificate: apmTLS.GetServerCert(),
+			ServerCA:          apmTLS.GetServerCa(),
+		}
+
+		cfg := config.Instrumentation{
+			Enabled:      true,
+			TLS:          iTLS,
+			Environment:  apmest.GetEnvironment(),
+			APIKey:       apmest.GetApiKey(),
+			SecretToken:  apmest.GetSecretToken(),
+			Hosts:        apmest.GetHosts(),
+			GlobalLabels: apmest.GetGlobalLabels(),
+		}
+		return cfg, nil
+	}
+	return config.Instrumentation{}, fmt.Errorf("unable to transform APMConfig to instrumentation")
+}