MXS-2078 Add IntervalTimer

IntervalTimer is used for accumulating intervals (i.e. durations) over sections of code.

maxutils/maxbase/src/stopwatch.cc

@@ -46,6 +46,27 @@ Duration StopWatch::restart()
     m_start = m_lap = now;
     return split;
 }
+
+IntervalTimer::IntervalTimer() : m_total(0)
+{
+}
+
+void IntervalTimer::start_interval()
+{
+    m_last_start = Clock::now();
+}
+
+void IntervalTimer::end_interval()
+{
+    m_total += Clock::now() - m_last_start;
+    // reset to make it easier to spot usage bugs, like calling end_interval(); end_interval();
+    m_last_start = TimePoint();
+}
+
+Duration IntervalTimer::total() const
+{
+    return m_total;
+}
 }   // maxbase
 
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