feat: Add Prometheus service client with instant query execution, alert retrieval, and health checking

- Add PrometheusService struct with baseURL/http.Client (10s timeout) and PROMETHEUS_URL env config (default: http://prometheus:9090)
- Add Available method with /-/ready endpoint health check
- Add Query method with instant PromQL execution via /api/v1/query, URL escaping, error handling
- Add promResponse/promVectorData/PromVectorSample structs for query result parsing with metric/value fields

apps/api-server/internal/monitoring/prometheus.go

@@ -0,0 +1,133 @@
+package monitoring
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"time"
+)
+
+// PrometheusService queries a Prometheus HTTP API for real system metrics
+// and alerts. Configure via PROMETHEUS_URL (e.g. http://prometheus:9090).
+type PrometheusService struct {
+	baseURL string
+	http    *http.Client
+}
+
+func NewPrometheusService() *PrometheusService {
+	base := os.Getenv("PROMETHEUS_URL")
+	if base == "" {
+		base = "http://prometheus:9090"
+	}
+	return &PrometheusService{
+		baseURL: strings.TrimRight(base, "/"),
+		http:    &http.Client{Timeout: 10 * time.Second},
+	}
+}
+
+// Available reports whether the configured Prometheus instance is reachable.
+func (p *PrometheusService) Available(ctx context.Context) bool {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, p.baseURL+"/-/ready", nil)
+	if err != nil {
+		return false
+	}
+	resp, err := p.http.Do(req)
+	if err != nil {
+		return false
+	}
+	defer resp.Body.Close()
+	return resp.StatusCode >= 200 && resp.StatusCode < 300
+}
+
+type promResponse struct {
+	Status string          `json:"status"`
+	Data   json.RawMessage `json:"data"`
+	Error  string          `json:"error,omitempty"`
+}
+
+type PromVectorSample struct {
+	Metric map[string]string `json:"metric"`
+	Value  [2]any            `json:"value"`
+}
+
+type promVectorData struct {
+	ResultType string             `json:"resultType"`
+	Result     []PromVectorSample `json:"result"`
+}
+
+// Query runs an instant PromQL query.
+func (p *PrometheusService) Query(ctx context.Context, promQL string) ([]PromVectorSample, error) {
+	u := fmt.Sprintf("%s/api/v1/query?query=%s", p.baseURL, url.QueryEscape(promQL))
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := p.http.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	b, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return nil, fmt.Errorf("prometheus query failed: %d %s", resp.StatusCode, string(b))
+	}
+	var out promResponse
+	if err := json.Unmarshal(b, &out); err != nil {
+		return nil, err
+	}
+	if out.Status != "success" {
+		return nil, fmt.Errorf("prometheus query error: %s", out.Error)
+	}
+	var data promVectorData
+	if err := json.Unmarshal(out.Data, &data); err != nil {
+		return nil, err
+	}
+	return data.Result, nil
+}
+
+// Alert represents an active Prometheus alert.
+type PromAlert struct {
+	Labels      map[string]string `json:"labels"`
+	Annotations map[string]string `json:"annotations"`
+	State       string            `json:"state"`
+	ActiveAt    time.Time         `json:"activeAt"`
+	Value       string            `json:"value"`
+}
+
+type promAlertsData struct {
+	Alerts []PromAlert `json:"alerts"`
+}
+
+// Alerts returns currently firing/pending alerts.
+func (p *PrometheusService) Alerts(ctx context.Context) ([]PromAlert, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, p.baseURL+"/api/v1/alerts", nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := p.http.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	b, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return nil, fmt.Errorf("prometheus alerts failed: %d %s", resp.StatusCode, string(b))
+	}
+	var out promResponse
+	if err := json.Unmarshal(b, &out); err != nil {
+		return nil, err
+	}
+	if out.Status != "success" {
+		return nil, fmt.Errorf("prometheus alerts error: %s", out.Error)
+	}
+	var data promAlertsData
+	if err := json.Unmarshal(out.Data, &data); err != nil {
+		return nil, err
+	}
+	return data.Alerts, nil
+}