Profiler: Add histogram for sample counts

Previously Profiler would use the stack depth to draw the timeline
graphs. This is not an accurate representation of whether a thread
is "busy" or not. Instead this updates the timelines to use the
sample count.

Userland/DevTools/Profiler/Histogram.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+
+template<typename Timestamp = u64, typename Value = u16, size_t inline_capacity = 4096>
+class Histogram {
+public:
+    Histogram(Timestamp start, Timestamp end, size_t bucket_count)
+        : m_start(start)
+        , m_end(end)
+    {
+        m_buckets.resize(bucket_count);
+    }
+
+    void insert(Timestamp const& timestamp, Value value = 1)
+    {
+        VERIFY(timestamp >= m_start && timestamp <= m_end);
+        auto bucket = (timestamp - m_start) * (m_buckets.size() - 1) / (m_end - m_start);
+        m_buckets[bucket] += value;
+    }
+
+    Value at(size_t index) const
+    {
+        return m_buckets[index];
+    }
+
+    size_t size() const
+    {
+        return m_buckets.size();
+    }
+
+private:
+    Timestamp m_start { 0 };
+    Timestamp m_end { 0 };
+    Vector<Value, inline_capacity> m_buckets;
+};