MM-61887: Log the userID if a metric exceeds the last histogram bucket (#29448)

We create a custom histogram metric that logs the userID
when the observed value is greater or equal to the last bucket value.

This allows us to start tracking the slowest users of a system
while at the same time not polluting the Prometheus metrics
by storing a userID for every observation.

https://mattermost.atlassian.net/browse/MM-61887

```release-note
NONE
```

server/enterprise/metrics/histogram_test.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.enterprise for license information.
+
+package metrics
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
+	"github.com/mattermost/mattermost/server/v8/channels/api4"
+	"github.com/mattermost/mattermost/server/v8/channels/testlib"
+)
+
+func TestWrappedObserver(t *testing.T) {
+	th := api4.Setup(t)
+	defer th.TearDown()
+
+	h := NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: MetricsNamespace,
+		Subsystem: MetricsSubsystemClientsWeb,
+		Name:      "test",
+		Buckets:   []float64{0, 5, 10},
+	}, []string{"l1"}, th.TestLogger)
+
+	h.With(prometheus.Labels{"l1": "hello"}, th.BasicUser.Id).Observe(6)
+	require.NoError(t, th.TestLogger.Flush())
+	testlib.AssertNoLog(t, th.LogBuffer, mlog.LvlWarn.Name, "Metric observation exceeded.")
+
+	h.With(prometheus.Labels{"l1": "hello"}, th.BasicUser.Id).Observe(10)
+	require.NoError(t, th.TestLogger.Flush())
+	testlib.AssertLog(t, th.LogBuffer, mlog.LvlWarn.Name, "Metric observation exceeded.")
+}