time: add update_monotonic to RateEstimator

update_monotonic does not reset the counter if a new count is smaller than
earlier counts.  Useful when consuming an unsorted stream of eveent counts.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/time.h

@@ -81,6 +81,9 @@ namespace crucible {
 		// Write count
 		void update(uint64_t new_count);
 
+		// Ignore counts that go backwards
+		void update_monotonic(uint64_t new_count);
+
 		// Read count
 		uint64_t count() const;