feat(tracing): add OpenTelemetry instrumentation to API gateway

- Add internal/tracing package with OTel tracer initialization
- HTTP middleware for server-side tracing (request/response attributes)
- Transport wrapper for client-side upstream call tracing
- Update proxy to use tracing transport
- Add OTEL_* env vars to k8s deployment

Traces flow: api-gateway -> otel-collector -> tempo -> grafana

internal/tracing/transport.go

@@ -0,0 +1,76 @@
+package tracing
+
+import (
+	"net/http"
+	"time"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/propagation"
+	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
+	"go.opentelemetry.io/otel/trace"
+)
+
+// Transport wraps an http.RoundTripper with tracing.
+type Transport struct {
+	base http.RoundTripper
+}
+
+// NewTransport creates a new tracing transport wrapper.
+func NewTransport(base http.RoundTripper) *Transport {
+	if base == nil {
+		base = http.DefaultTransport
+	}
+	return &Transport{base: base}
+}
+
+// RoundTrip implements http.RoundTripper with tracing.
+func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	ctx := req.Context()
+	tracer := otel.Tracer(tracerName)
+	propagator := otel.GetTextMapPropagator()
+
+	// Start client span
+	spanName := "HTTP " + req.Method + " " + req.URL.Host + req.URL.Path
+	ctx, span := tracer.Start(ctx, spanName,
+		trace.WithSpanKind(trace.SpanKindClient),
+		trace.WithAttributes(
+			semconv.HTTPRequestMethodKey.String(req.Method),
+			semconv.URLFull(req.URL.String()),
+			semconv.ServerAddress(req.URL.Host),
+			attribute.String("upstream.name", req.URL.Host),
+		),
+	)
+	defer span.End()
+
+	// Inject trace context into outgoing request headers
+	propagator.Inject(ctx, propagation.HeaderCarrier(req.Header))
+
+	// Perform the request
+	start := time.Now()
+	resp, err := t.base.RoundTrip(req.WithContext(ctx))
+	duration := time.Since(start)
+
+	// Record timing
+	span.SetAttributes(attribute.Float64("http.request.duration_ms", float64(duration.Milliseconds())))
+
+	if err != nil {
+		span.RecordError(err)
+		span.SetStatus(codes.Error, err.Error())
+		return nil, err
+	}
+
+	// Record response attributes
+	span.SetAttributes(
+		semconv.HTTPResponseStatusCode(resp.StatusCode),
+	)
+
+	if resp.StatusCode >= 400 {
+		span.SetStatus(codes.Error, http.StatusText(resp.StatusCode))
+	} else {
+		span.SetStatus(codes.Ok, "")
+	}
+
+	return resp, nil
+}