Document count_values aggregator.

content/docs/querying/operators.md

@@ -185,6 +185,7 @@ vector of fewer elements with aggregated values:
 * `stddev` (calculate population standard deviation over dimensions)
 * `stdvar` (calculate population standard variance over dimensions)
 * `count` (count number of elements in the vector)
+* `count_values` (count number of elements with the same value)
 * `bottomk` (smallest k elements by sample value)
 * `topk` (largest k elements by sample value)
 
@@ -193,12 +194,18 @@ or preserve distinct dimensions by including a `without` or `by` clause.
 
     <aggr-op>([parameter,] <vector expression>) [without|by (<label list>)] [keep_common]
 
-`parameter` is only required for `topk` and `bottomk`. `without` removes the
-listed labels from the result vector, while all other labels are preserved the
-output. `by` does the opposite and drops labels that are not listed in the `by`
-clause, even if their label values are identical between all elements of the
-vector. The `keep_common` clause allows keeping those extra labels (labels that
-are identical between elements, but not in the `by` clause).
+`parameter` is only required for `count_values`,`topk` and `bottomk`. `without`
+removes the listed labels from the result vector, while all other labels are
+preserved the output. `by` does the opposite and drops labels that are not
+listed in the `by` clause, even if their label values are identical between all
+elements of the vector. The `keep_common` clause allows keeping those extra
+labels (labels that are identical between elements, but not in the `by`
+clause).
+
+`count_values` outputs one time series per unique sample value. Each series has
+an additional label. The name of that label is given by the aggregation
+parameter, and the label value is the unique sample value.  The value of each
+time series is the number of times that sample value was present.
 
 `topk` and `bottomk` are different from other aggregators in that a subset of
 the input samples, including the original labels, are returned in the result
@@ -217,6 +224,10 @@ applications, we could simply write:
 
     sum(http_requests_total)
 
+To count the number of binaries running each build version we could write:
+
+    count_values("version", build_version)
+
 To get the 5 largest HTTP requests counts across all instances we could write:
 
     topk(5, http_requests_total)