perf(monitoring): cache hostname and bound the label-key cache

get_metrics_name called os.Hostname on every metrics call and keyed a
never-evicted cache by label values. The hostname is now resolved once
and the cache is keyed by label names, so it cannot grow with high
cardinality values. The metrics listener also honours a graceful
shutdown context.

monitoring/helpers_test.go

@@ -1,10 +1,14 @@
 package libpack_monitoring
 
 import (
+	"strings"
+	"sync"
+	"sync/atomic"
 	"testing"
 
 	libpack_config "github.com/lukaszraczylo/graphql-monitoring-proxy/config"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestGetMetricsName(t *testing.T) {
@@ -201,6 +205,75 @@ func TestEnsureDefaultLabels(t *testing.T) {
 	}
 }
 
+// TestGetPodName_HostnameResolvedOnce verifies getPodName() resolves the
+// hostname exactly once (via podNameOnce) no matter how many times it is
+// called, including concurrently. Repeated os.Hostname() syscalls on every
+// metrics call were the dominant per-request cost this caching removes.
+func TestGetPodName_HostnameResolvedOnce(t *testing.T) {
+	origFn := hostnameFunc
+	t.Cleanup(func() {
+		// Restore the real hostname resolver and reset to a fresh, unfired
+		// Once (never copy an existing sync.Once - it must not be copied by
+		// value once it may have been used). The next getPodName() call will
+		// re-resolve the real hostname and re-cache it for later tests.
+		hostnameFunc = origFn
+		podNameOnce = sync.Once{}
+	})
+
+	podNameOnce = sync.Once{}
+	var callCount int32
+	hostnameFunc = func() (string, error) {
+		atomic.AddInt32(&callCount, 1)
+		return "cached-test-host", nil
+	}
+
+	// Sequential calls must all observe the cached value.
+	first := getPodName()
+	second := getPodName()
+	assert.Equal(t, "cached-test-host", first)
+	assert.Equal(t, first, second)
+	assert.Equal(t, int32(1), atomic.LoadInt32(&callCount), "hostnameFunc must be invoked exactly once")
+
+	// Concurrent calls must not re-invoke hostnameFunc either.
+	var wg sync.WaitGroup
+	for i := 0; i < 50; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			assert.Equal(t, "cached-test-host", getPodName())
+		}()
+	}
+	wg.Wait()
+
+	assert.Equal(t, int32(1), atomic.LoadInt32(&callCount), "hostnameFunc must still be invoked exactly once after concurrent calls")
+}
+
+// TestGetSortedKeysCache_SharedAcrossValues verifies the sortedLabelKeysCache
+// is keyed by the sorted set of label NAMES only. Two maps sharing the same
+// keys but different values must collapse into a single cache entry rather
+// than growing the cache per distinct value combination (unbounded
+// cardinality bomb).
+func TestGetSortedKeysCache_SharedAcrossValues(t *testing.T) {
+	labelsA := map[string]string{"cache_key_test_a": "value1", "cache_key_test_b": "value2"}
+	labelsB := map[string]string{"cache_key_test_a": "totally-different", "cache_key_test_b": "also-different"}
+
+	keysA := getSortedKeys(labelsA)
+	keysB := getSortedKeys(labelsB)
+
+	require.Equal(t, []string{"cache_key_test_a", "cache_key_test_b"}, keysA)
+	require.Equal(t, keysA, keysB)
+
+	// Same underlying slice (same cache entry) must be returned for both
+	// maps, proving the cache did not create a second, value-keyed entry.
+	assert.Same(t, &keysA[0], &keysB[0])
+
+	// The cache entry itself must be reachable only via a names-only key.
+	namesKey := strings.Join(keysA, ";")
+	cached, ok := sortedLabelKeysCache.m.Load(namesKey)
+	require.True(t, ok)
+	assert.Equal(t, keysA, cached)
+}
+
 func TestLabelsToString(t *testing.T) {
 	tests := []struct {
 		labels   map[string]string