feat(ratelimit): track per-window token usage and utilization

Poll /api/oauth/usage every 5 min and extract utilization from
/v1/messages response headers for real-time updates. Track proxy
tokens in/out per rate limit window (5h/7d), resetting on window
change. Expose as OTel observable gauges for Grafana dashboards.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/telemetry/telemetry.go

@@ -5,6 +5,7 @@ import (
 	"io"
 
 	"github.com/fujin/anthropic-proxy/internal/config"
+	"github.com/fujin/anthropic-proxy/internal/ratelimit"
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc"
 	"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc"
@@ -21,7 +22,7 @@ import (
 // so metrics can be recorded in-process. When cfg.ExportEnabled(), OTLP gRPC
 // exporters are additionally configured to push to the LGTM stack.
 // Returns a shutdown function and an optional io.Writer for the log bridge.
-func Setup(ctx context.Context, cfg config.TelemetryConfig) (shutdown func(context.Context) error, logWriter io.Writer, err error) {
+func Setup(ctx context.Context, cfg config.TelemetryConfig, tracker *ratelimit.Tracker) (shutdown func(context.Context) error, logWriter io.Writer, err error) {
 	res, err := resource.New(ctx,
 		resource.WithAttributes(
 			semconv.ServiceName(cfg.ServiceName),
@@ -36,7 +37,7 @@ func Setup(ctx context.Context, cfg config.TelemetryConfig) (shutdown func(conte
 		// instruments are valid (they just don't export anywhere).
 		mp := sdkmetric.NewMeterProvider(sdkmetric.WithResource(res))
 		otel.SetMeterProvider(mp)
-		InitMetrics(mp.Meter(cfg.ServiceName))
+		InitMetrics(mp.Meter(cfg.ServiceName), tracker)
 		return func(ctx context.Context) error { return mp.Shutdown(ctx) }, nil, nil
 	}
 
@@ -74,7 +75,7 @@ func Setup(ctx context.Context, cfg config.TelemetryConfig) (shutdown func(conte
 		sdkmetric.WithResource(res),
 	)
 	otel.SetMeterProvider(mp)
-	InitMetrics(mp.Meter(cfg.ServiceName))
+	InitMetrics(mp.Meter(cfg.ServiceName), tracker)
 
 	// Log exporter
 	logExp, err := otlploggrpc.New(ctx, logOpts...)