gochat/backend/internal/handler/metrics_handler.go

package handler

import (
	"fmt"
	"net/http"
	"runtime"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/gin-gonic/gin"
)

var durationBuckets = [...]float64{0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5}

type requestMetricKey struct {
	Method string
	Route  string
	Status int
}

type requestMetric struct {
	Count   uint64
	Errors  uint64
	Sum     float64
	Buckets [len(durationBuckets)]uint64
}

// HTTPMetrics records bounded route-template labels without a Prometheus client dependency.
type HTTPMetrics struct {
	startTime time.Time
	mu        sync.RWMutex
	requests  map[requestMetricKey]requestMetric
}

func NewHTTPMetrics(startTime time.Time) *HTTPMetrics {
	return &HTTPMetrics{startTime: startTime, requests: make(map[requestMetricKey]requestMetric)}
}

func (m *HTTPMetrics) Middleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		started := time.Now()
		c.Next()
		route := c.FullPath()
		if route == "" {
			route = "unmatched"
		}
		m.observe(requestMetricKey{Method: c.Request.Method, Route: route, Status: c.Writer.Status()}, time.Since(started).Seconds())
	}
}

func (m *HTTPMetrics) observe(key requestMetricKey, seconds float64) {
	m.mu.Lock()
	metric := m.requests[key]
	metric.Count++
	metric.Sum += seconds
	if key.Status >= http.StatusInternalServerError {
		metric.Errors++
	}
	for i, boundary := range durationBuckets {
		if seconds <= boundary {
			metric.Buckets[i]++
		}
	}
	m.requests[key] = metric
	m.mu.Unlock()
}

func (m *HTTPMetrics) Handler() gin.HandlerFunc {
	return func(c *gin.Context) {
		var memory runtime.MemStats
		runtime.ReadMemStats(&memory)
		keys, snapshot := m.snapshot()

		var output strings.Builder
		output.WriteString("# HELP http_requests_total Total HTTP requests\n# TYPE http_requests_total counter\n")
		output.WriteString("# HELP http_request_errors_total Total HTTP 5xx responses\n# TYPE http_request_errors_total counter\n")
		output.WriteString("# HELP http_request_duration_seconds HTTP request duration\n# TYPE http_request_duration_seconds histogram\n")
		for _, key := range keys {
			metric := snapshot[key]
			labels := requestLabels(key)
			fmt.Fprintf(&output, "http_requests_total{%s} %d\n", labels, metric.Count)
			fmt.Fprintf(&output, "http_request_errors_total{%s} %d\n", labels, metric.Errors)
			for i, boundary := range durationBuckets {
				fmt.Fprintf(&output, "http_request_duration_seconds_bucket{%s,le=%q} %d\n", labels, strconv.FormatFloat(boundary, 'g', -1, 64), metric.Buckets[i])
			}
			fmt.Fprintf(&output, "http_request_duration_seconds_bucket{%s,le=\"+Inf\"} %d\n", labels, metric.Count)
			fmt.Fprintf(&output, "http_request_duration_seconds_sum{%s} %s\n", labels, strconv.FormatFloat(metric.Sum, 'g', -1, 64))
			fmt.Fprintf(&output, "http_request_duration_seconds_count{%s} %d\n", labels, metric.Count)
		}

		output.WriteString("# HELP gochat_go_goroutines Number of goroutines currently running\n# TYPE gochat_go_goroutines gauge\n")
		output.WriteString("# HELP gochat_go_memory_alloc_bytes Bytes of allocated heap objects\n# TYPE gochat_go_memory_alloc_bytes gauge\n")
		output.WriteString("# HELP gochat_go_memory_sys_bytes Bytes obtained from system\n# TYPE gochat_go_memory_sys_bytes gauge\n")
		output.WriteString("# HELP gochat_uptime_seconds Application uptime in seconds\n# TYPE gochat_uptime_seconds gauge\n")
		fmt.Fprintf(&output, "gochat_go_goroutines %d\n", runtime.NumGoroutine())
		fmt.Fprintf(&output, "gochat_go_memory_alloc_bytes %d\n", memory.Alloc)
		fmt.Fprintf(&output, "gochat_go_memory_sys_bytes %d\n", memory.Sys)
		fmt.Fprintf(&output, "gochat_go_memory_total_alloc_bytes %d\n", memory.TotalAlloc)
		fmt.Fprintf(&output, "gochat_go_gc_pause_total_ns %d\n", memory.PauseTotalNs)
		fmt.Fprintf(&output, "gochat_go_gc_count %d\n", memory.NumGC)
		fmt.Fprintf(&output, "gochat_uptime_seconds %s\n", strconv.FormatFloat(time.Since(m.startTime).Seconds(), 'f', 3, 64))
		fmt.Fprintf(&output, "gochat_threads_count %d\n", runtime.NumCPU())

		c.Data(http.StatusOK, "text/plain; version=0.0.4; charset=utf-8", []byte(output.String()))
	}
}

func (m *HTTPMetrics) snapshot() ([]requestMetricKey, map[requestMetricKey]requestMetric) {
	m.mu.RLock()
	snapshot := make(map[requestMetricKey]requestMetric, len(m.requests))
	keys := make([]requestMetricKey, 0, len(m.requests))
	for key, metric := range m.requests {
		keys = append(keys, key)
		snapshot[key] = metric
	}
	m.mu.RUnlock()
	sort.Slice(keys, func(i, j int) bool {
		left, right := keys[i], keys[j]
		if left.Route != right.Route {
			return left.Route < right.Route
		}
		if left.Method != right.Method {
			return left.Method < right.Method
		}
		return left.Status < right.Status
	})
	return keys, snapshot
}

func requestLabels(key requestMetricKey) string {
	return fmt.Sprintf(`method="%s",route="%s",status="%s"`, escapeLabel(key.Method), escapeLabel(key.Route), strconv.Itoa(key.Status))
}

func escapeLabel(value string) string {
	value = strings.ReplaceAll(value, `\`, `\\`)
	value = strings.ReplaceAll(value, "\n", `\n`)
	return strings.ReplaceAll(value, `"`, `\"`)
}

// PrometheusHandler is retained for callers that only need runtime metrics.
func PrometheusHandler(startTime time.Time) gin.HandlerFunc {
	return NewHTTPMetrics(startTime).Handler()
}

func formatGauge(name string, value uint64) string   { return name + " " + formatValue(value) }
func formatCounter(name string, value uint64) string { return name + " " + formatValue(value) }
func formatValue(value uint64) string                { return strconv.FormatUint(value, 10) }