Remove RelatedHistogramSet.

docs/how-to-write-metrics.md

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      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file.
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 # How to Write Metrics
 
 Timeline-Based Measurement v2 is a system for computing metrics from traces.
 
 A TBM2 metric is a Javascript function that takes a trace Model and produces
@@ skipping 131 matching lines @@
 
  * [DateRange](/tracing/tracing/value/diagnostics/date_range.html):
    This is a Range of Dates. It cannot be empty, but the minDate could be the
    same as the maxDate. Telemetry automatically adds 2 shared DateRanges to all
    results: 'benchmarkStart' and 'traceStart'.
 
    ![](/docs/images/how-to-write-metrics-date-range.png)
 
 ### Histogram Relationship Diagnostics
 
- * [RelatedHistogramSet](/tracing/tracing/value/diagnostics/related_histogram_set.html):
-   These are Sets of references to other Histograms. Visually, they are a set
-   of HTML links which, when clicked, select the contained Histograms. The text
-   content of the HTML link is the name of the referenced Histogram.
-
-   ![](/docs/images/how-to-write-metrics-related-histogram-set.png)
-
  * [RelatedHistogramMap](/tracing/tracing/value/diagnostics/related_histogram_map.html):
    These are Maps from strings to references to other Histograms. Visually, they
-   are a set of HTML links similar to RelatedHistogramSet, but the text content of
-   the link is the Map's string key instead of the Histogram's name. One example
-   application is when a Histogram was produced not directly by a metric, but
-   rather by merging together other Histograms, then it will have a
-   RelatedHistogramMap named 'mergedFrom' that refers to the Histograms that were
-   merged by their grouping key, e.g. the telemetry story name.
+   are a set of HTML links where the text content of the link is the Map's
+   string key instead of the Histogram's name. One example application is when a
+   Histogram was produced not directly by a metric, but rather by merging
+   together other Histograms, then it will have a RelatedHistogramMap named
+   'mergedFrom' that refers to the Histograms that were merged by their grouping
+   key, e.g. the telemetry story name.
 
    ![](/docs/images/how-to-write-metrics-related-histogram-map.png)
 
  * [RelatedHistogramBreakdown](/tracing/tracing/value/diagnostics/related_histogram_breakdown.html):
    Structurally, this is a RelatedHistogramMap, but conceptually and visually, this
    is a Breakdown. Whereas Breakdown's stacked bar chart derives its data from
    the numbers contained explicitly in the Breakdown, a
    RelatedHistogramBreakdown's stacked
    bar chart derives its data from the referenced Histograms' sums.
 
@@ skipping 20 matching lines @@
    ![](/docs/images/how-to-write-metrics-buildbot.png)
 
 ### Other Diagnostics
 
  * [Scalar](/tracing/tracing/value/diagnostics/scalar.html):
    Metrics must not use this, since it is incapable of being merged. It is
    mentioned here for completeness. It wraps a Scalar, which is just a
    unitted number. This is only to allow Histograms in other parts of the trace
    viewer to display number sample diagnostics more intelligently than
    GenericSet can. If a metric wants to display number sample diagnostics
-   intelligently, then it should use RelatedHistogramSet or RelatedHistogramMap;
-   if it does not want to monitor changes in those numbers, then the TBM2
-   maintainers can add a HistogramDiagnostic that supports merging.
+   intelligently, then it should use RelatedHistogramMap; if it does not want to
+   monitor changes in those numbers, then the TBM2 maintainers can add a
+   HistogramDiagnostic that supports merging.
 
 
 ## Consumers of Histograms
 
 Histograms are consumed by
 
  * [histogram-set-table](/tracing/tracing/value/ui/histogram_set_table.html) in
    both results2.html and the Metrics side panel in trace viewer,
  * the [dashboard](https://chromeperf.appspot.com) indirectly via their statistics.
 
@@ skipping 35 matching lines @@
   * Instead of having one category for all story grouping keys, each grouping
     individual grouping key may be listed as a category. For example, in Page
     Cycler v2 benchmarks, the "cache_temperature" grouping key would be
     displayed as a category.
 * Choosing groups builds a hierarchy of histograms that is filled in by merging
   histograms from the bottom up. Expanding the rows of histogram-set-table, any
   leaf nodes are histograms that were loaded, and their ancestors are computed by
   merging.
 * histogram-set-table uses the "label" property of TelemetryInfo to define the
   columns of the table.