From 3f412ab1ea99a345865ce3ca4120a9ff9e78b038 Mon Sep 17 00:00:00 2001
From: "google-labs-jules[bot]"
 <161369871+google-labs-jules[bot]@users.noreply.github.com>
Date: Sun, 15 Mar 2026 18:10:11 +0000
Subject: [PATCH] feat: Add basic metrics via expvar to health server

This commit implements basic application metrics via the `expvar` package by exposing it on the `/debug/vars` endpoint within the custom HTTP server in `pkg/health/server.go`. It fulfills the "Basic Metrics Implementation" requirement under the "Ultimate Visibility" framework.

The `docs/design/ETL_TODO.md` documentation has also been updated to mark this task as completed.

Co-authored-by: hobbyistlabs-coder <267281733+hobbyistlabs-coder@users.noreply.github.com>
---
 docs/design/ETL_TODO.md | 2 +-
 pkg/health/server.go    | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/docs/design/ETL_TODO.md b/docs/design/ETL_TODO.md
index c1c2178bc..9ee73b3d9 100644
--- a/docs/design/ETL_TODO.md
+++ b/docs/design/ETL_TODO.md
@@ -5,7 +5,7 @@ This document tracks the tasks required to implement the "Ultimate Visibility" E
 ## 1. Extract (Ingestion & Telemetry Collection)
 
 - [ ] **Structured Logging:** Ensure `zerolog` is used consistently across the codebase for structured JSON logging. Add context to logs where missing (session IDs, tool inputs/outputs).
-- [ ] **Basic Metrics Implementation:** Introduce a metrics package (e.g., using `expvar` or a Prometheus client) to expose basic application metrics.
+- [x] **Basic Metrics Implementation:** Introduce a metrics package (e.g., using `expvar` or a Prometheus client) to expose basic application metrics.
 - [x] **Goroutine Tracking:** Implement a metric to track the number of active Goroutines.
 - [x] **Memory Tracking:** Implement a metric to track heap allocation and GC pauses.
 - [ ] **AgentLoop Telemetry:** Add specific instrumentation to the `AgentLoop` (iteration duration, tool execution duration, failure counts).
diff --git a/pkg/health/server.go b/pkg/health/server.go
index 632e4c346..b3f1c5a30 100644
--- a/pkg/health/server.go
+++ b/pkg/health/server.go
@@ -3,6 +3,7 @@ package health
 import (
 	"context"
 	"encoding/json"
+	"expvar"
 	"fmt"
 	"maps"
 	"net/http"
@@ -41,6 +42,7 @@ func NewServer(host string, port int) *Server {
 
 	mux.HandleFunc("/health", s.healthHandler)
 	mux.HandleFunc("/ready", s.readyHandler)
+	mux.Handle("/debug/vars", expvar.Handler())
 
 	addr := fmt.Sprintf("%s:%d", host, port)
 	s.server = &http.Server{
@@ -162,6 +164,7 @@ func (s *Server) readyHandler(w http.ResponseWriter, r *http.Request) {
 func (s *Server) RegisterOnMux(mux *http.ServeMux) {
 	mux.HandleFunc("/health", s.healthHandler)
 	mux.HandleFunc("/ready", s.readyHandler)
+	mux.Handle("/debug/vars", expvar.Handler())
 }
 
 func statusString(ok bool) string {