Metrics Documentation - Add Instruction for How To Unit Test

tools/metrics/histograms/README.md

 # Histogram Guidelines
 
 This document gives the best practices on how to use histograms in code and how
 to document the histograms for the dashboards.  There are three general types
 of histograms: enumerated histograms, count histograms (for arbitrary numbers),
 and sparse histograms (for anything when the precision is important over a wide
 range and/or the range is not possible to specify a priori).
 
 [TOC]
 
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 pieces of information need to be logged together. For example, a developer may
 be interested in the counts of features X and Y based on whether a user is in
 state A or B. In this case, they want to know the count of X under state A,
 as well as the other three permutations.
 
 There is no general purpose solution for this type of analysis. We suggest
 using the workaround of using an enum of length MxN, where you log each unique
 pair {state, feature} as a separate entry in the same enum. If this causes a
 large explosion in data (i.e. >100 enum entries), a [sparse histogram](#When-To-Use-Sparse-Histograms) may be appropriate. If you are unsure of the best way to proceed, please contact someone from the OWNERS file.
 
-### Testing
+## Testing
 
 Test your histograms using `chrome://histograms`.  Make sure they're being
 emitted to when you expect and not emitted to at other times. Also check that
 the values emitted to are correct.  Finally, for count histograms, make sure
 that buckets capture enough precision for your needs over the range.
 
-### Revising Histograms
+In addition to testing interactively, you can have unit tests examine the
+values emitted to histograms.  See [histogram_tester.h](https://cs.chromium.org/chromium/src/base/test/histogram_tester.h)
+for details.
 
-If you're changing the semantics of a histogram (when it's emitted, what
-buckets mean, etc.), make it into a new histogram with a new name.  Otherwise
-the "Everything" view on the dashboard will be mixing two different
+## Revising Histograms
+
+When changing the semantics of a histogram (when it's emitted, what buckets
+mean, etc.), make it into a new histogram with a new name.  Otherwise the
+"Everything" view on the dashboard will be mixing two different
 interpretations of the data and make no sense.
 
-### Deleting Histograms
+## Deleting Histograms
 
 Please delete the code that emits to histograms that are no longer needed.
 Histograms take up memory.  Cleaning up histograms that you no longer care about
 is good!  But see the note below on [Deleting Histogram Entries](#Deleting-Histogram-Entries).
 
 ## Documenting Histograms
 
 ### Add Histogram and Documentation in the Same Changelist
 
 If possible, please add the [histograms.xml](./histograms.xml) description in
@@ skipping 50 matching lines @@
 Sparse histograms are well suited for recording counts of exact sample values
 that are sparsely distributed over a large range.
 
 The implementation uses a lock and a map, whereas other histogram types use a
 vector and no lock. It is thus more costly to add values to, and each value
 stored has more overhead, compared to the other histogram types. However it
 may be more efficient in memory if the total number of sample values is small
 compared to the range of their values.
 
 For more information, see [sparse_histograms.h](https://cs.chromium.org/chromium/src/base/metrics/sparse_histogram.h).