feat(metrics): bound HTTP path labels, add reset, custom push endpoint and JSON pull

- normalize the HTTP path label (ServeMux pattern, custom normalizer, ID
  collapsing) and cap distinct values via HTTPMaxPaths (default 1024)
- add Reset, PushAndReset, ResetHandler and reset-on-push options
- add POST push to a custom endpoint (text or json) with optional reset
- add JSONHandler for JSON pull
- honour Config.Enabled; log Pushgateway push failures

pkg/metrics/endpoint.go

@@ -0,0 +1,189 @@
+package metrics
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
+	"github.com/prometheus/common/expfmt"
+
+	"github.com/bitechdev/ResolveSpec/pkg/logger"
+)
+
+const textContentType = "text/plain; version=0.0.4; charset=utf-8"
+
+// endpointPusher POSTs gathered metrics to a user-configured HTTP endpoint.
+type endpointPusher struct {
+	url       string
+	format    string
+	headers   map[string]string
+	client    *http.Client
+	resetOnOK bool
+	provider  *PrometheusProvider
+	gatherer  prometheus.Gatherer
+	stopOnce  sync.Once
+	stopCh    chan struct{}
+	startedMu sync.Mutex
+	started   bool
+}
+
+func newEndpointPusher(cfg *Config, p *PrometheusProvider) *endpointPusher {
+	return &endpointPusher{
+		url:       cfg.PushEndpointURL,
+		format:    cfg.PushEndpointFormat,
+		headers:   cfg.PushEndpointHeaders,
+		client:    &http.Client{Timeout: time.Duration(cfg.PushEndpointTimeout) * time.Second},
+		resetOnOK: cfg.PushEndpointResetOnSuccess,
+		provider:  p,
+		gatherer:  prometheus.DefaultGatherer,
+		stopCh:    make(chan struct{}),
+	}
+}
+
+func (e *endpointPusher) start(interval time.Duration) {
+	e.startedMu.Lock()
+	defer e.startedMu.Unlock()
+	if e.started {
+		return
+	}
+	e.started = true
+	go func() {
+		t := time.NewTicker(interval)
+		defer t.Stop()
+		for {
+			select {
+			case <-t.C:
+				if err := e.push(context.Background()); err != nil {
+					logger.Warn("Failed to push metrics to endpoint %s: %v", e.url, err)
+				}
+			case <-e.stopCh:
+				return
+			}
+		}
+	}()
+}
+
+func (e *endpointPusher) stop() {
+	e.stopOnce.Do(func() { close(e.stopCh) })
+}
+
+func (e *endpointPusher) push(ctx context.Context) error {
+	mfs, err := e.gatherer.Gather()
+	if err != nil && len(mfs) == 0 {
+		return fmt.Errorf("gather metrics: %w", err)
+	}
+
+	body, contentType, err := encodeMetrics(mfs, e.format)
+	if err != nil {
+		return err
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, e.url, bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", contentType)
+	for k, v := range e.headers {
+		req.Header.Set(k, v)
+	}
+
+	resp, err := e.client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	snippet, _ := io.ReadAll(io.LimitReader(resp.Body, 512))
+	if resp.StatusCode < 200 || resp.StatusCode > 299 {
+		return fmt.Errorf("endpoint returned %s: %s", resp.Status, bytes.TrimSpace(snippet))
+	}
+
+	if e.resetOnOK {
+		e.provider.Reset()
+	}
+	return nil
+}
+
+func encodeMetrics(mfs []*dto.MetricFamily, format string) (body []byte, contentType string, err error) {
+	switch format {
+	case "json":
+		b, err := json.Marshal(toJSONFamilies(mfs))
+		return b, "application/json", err
+	case "", "text":
+		var buf bytes.Buffer
+		enc := expfmt.NewEncoder(&buf, expfmt.NewFormat(expfmt.TypeTextPlain))
+		for _, mf := range mfs {
+			if err := enc.Encode(mf); err != nil {
+				return nil, "", err
+			}
+		}
+		return buf.Bytes(), textContentType, nil
+	default:
+		return nil, "", fmt.Errorf("unsupported push endpoint format %q", format)
+	}
+}
+
+type jsonFamily struct {
+	Name    string       `json:"name"`
+	Help    string       `json:"help,omitempty"`
+	Type    string       `json:"type"`
+	Metrics []jsonMetric `json:"metrics"`
+}
+
+type jsonMetric struct {
+	Labels  map[string]string `json:"labels,omitempty"`
+	Value   *float64          `json:"value,omitempty"`
+	Count   *uint64           `json:"count,omitempty"`
+	Sum     *float64          `json:"sum,omitempty"`
+	Buckets []jsonBucket      `json:"buckets,omitempty"`
+}
+
+type jsonBucket struct {
+	UpperBound float64 `json:"le"`
+	Count      uint64  `json:"count"`
+}
+
+func toJSONFamilies(mfs []*dto.MetricFamily) []jsonFamily {
+	out := make([]jsonFamily, 0, len(mfs))
+	for _, mf := range mfs {
+		f := jsonFamily{Name: mf.GetName(), Help: mf.GetHelp(), Type: mf.GetType().String()}
+		for _, m := range mf.GetMetric() {
+			jm := jsonMetric{}
+			if len(m.GetLabel()) > 0 {
+				jm.Labels = make(map[string]string, len(m.GetLabel()))
+				for _, l := range m.GetLabel() {
+					jm.Labels[l.GetName()] = l.GetValue()
+				}
+			}
+			switch {
+			case m.Counter != nil:
+				v := m.Counter.GetValue()
+				jm.Value = &v
+			case m.Gauge != nil:
+				v := m.Gauge.GetValue()
+				jm.Value = &v
+			case m.Untyped != nil:
+				v := m.Untyped.GetValue()
+				jm.Value = &v
+			case m.Histogram != nil:
+				c, s := m.Histogram.GetSampleCount(), m.Histogram.GetSampleSum()
+				jm.Count, jm.Sum = &c, &s
+				for _, b := range m.Histogram.GetBucket() {
+					jm.Buckets = append(jm.Buckets, jsonBucket{UpperBound: b.GetUpperBound(), Count: b.GetCumulativeCount()})
+				}
+			case m.Summary != nil:
+				c, s := m.Summary.GetSampleCount(), m.Summary.GetSampleSum()
+				jm.Count, jm.Sum = &c, &s
+			}
+			f.Metrics = append(f.Metrics, jm)
+		}
+		out = append(out, f)
+	}
+	return out
+}