fix(circuitbreaker): guard nil monitoring to prevent boot panic

Starting with ENABLE_CIRCUIT_BREAKER=true panicked at boot: parseConfig -> initCircuitBreaker registers a metrics gauge, but cfg.Monitoring is assigned later by StartMonitoringServer, so the registration dereferenced a nil *MetricsSetup.

Add nil-receiver guards to the MetricsSetup registration methods (RegisterMetricsGauge/GaugeFunc/Counter, RegisterFloatCounter, RegisterMetricsSummary/Histogram), matching their existing 'return a dummy to prevent panics' contract. The guard is a no-op for non-nil receivers, so existing callers are unaffected. The breaker and the admin dashboard (which reads gobreaker state directly) keep working.

Add regression tests for the nil-receiver methods and for initCircuitBreaker with nil monitoring.

circuit_breaker_nil_monitoring_test.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+
+	libpack_logger "github.com/lukaszraczylo/graphql-monitoring-proxy/logging"
+	"github.com/stretchr/testify/require"
+)
+
+// TestInitCircuitBreaker_NilMonitoring_NoPanic is a regression test for a boot
+// panic: enabling the circuit breaker (ENABLE_CIRCUIT_BREAKER=true) crashed at
+// startup because parseConfig calls initCircuitBreaker before the monitoring
+// server assigns cfg.Monitoring (StartMonitoringServer runs later). The metrics
+// gauge registration then dereferenced a nil *MetricsSetup.
+//
+// The breaker (and the admin dashboard, which reads gobreaker state directly)
+// must initialise without panicking even when monitoring is not yet available.
+func TestInitCircuitBreaker_NilMonitoring_NoPanic(t *testing.T) {
+	origCfg := cfg
+	cbMutex.Lock()
+	origCb, origMetrics := cb, cbMetrics
+	cb, cbMetrics = nil, nil
+	cbMutex.Unlock()
+	t.Cleanup(func() {
+		cbMutex.Lock()
+		cb, cbMetrics = origCb, origMetrics
+		cbMutex.Unlock()
+		cfg = origCfg
+	})
+
+	cfg = &config{}
+	cfg.Logger = libpack_logger.New().SetOutput(&bytes.Buffer{})
+	cfg.Monitoring = nil // the production state when parseConfig runs
+	cfg.CircuitBreaker.Enable = true
+	cfg.CircuitBreaker.MaxFailures = 3
+	cfg.CircuitBreaker.Timeout = 5
+	cfg.CircuitBreaker.MaxRequestsInHalfOpen = 2
+
+	require.NotPanics(t, func() { initCircuitBreaker(cfg) },
+		"initCircuitBreaker must not panic when cfg.Monitoring is nil")
+	require.NotNil(t, cb, "circuit breaker must initialise even without monitoring")
+	require.NotNil(t, cbMetrics, "circuit breaker metrics manager must be created")
+}