allow activities to set hdr-digits

engine-docs/src/main/resources/docs-for-nb/reference/activity_parameters.md

@@ -360,9 +360,21 @@ In detail, the rendering appears as `0.0(A), 0.0(B), 0.0(C), 0.25(A),
 0.5(A), 0.5(B), 0.75(A)`, which yields `A B C A A B A` as the op
 sequence.
 
-This sequencer is most useful when you want a stable ordering of
-operation from a rich mix of statement types, where each operations is
-spaced as evenly as possible over time, and where it is not important to
-control the cycle-by-cycle sequencing of statements.
+This sequencer is most useful when you want a stable ordering of operation from a rich mix of statement types, where
+each operations is spaced as evenly as possible over time, and where it is not important to control the cycle-by-cycle
+sequencing of statements.
 
+## hdr-digits
 
+- `hdr-digits=3`
+- _default_: `4`
+- _required_: no
+- _dynamic_: no
+
+This parameter determines the number of significant digits used in all HDR histograms for metrics collected from this
+activity. The default of 4 allows 4 significant digits, which means *up to* 10000 distinct histogram buckets per named
+metric, per histogram interval. This does not mean that there _will be_ 10000 distinct buckets, but it means there could
+be if there is significant volume and variety in the measurements.
+
+If you are running a scenario that creates many activities, then you can set `hdr-digits=1` on some of them to save
+client resources.

engine-docs/src/main/resources/docs-for-nb/reference/command_line.md

@@ -160,7 +160,7 @@ you can do so this way:
 
 The default is 4 digits, which creates 10000 equisized histogram buckets for every named metric in every reporting
 interval. For longer running test or for test which do not require this level of precision in metrics, you can set this
-down to 3 or 2.
+down to 3 or 2. Note that this only sets the global default. Each activity can also override this value.
 
 
 Enlist engineblock to stand up your metrics infrastructure using a local docker runtime: