Implement a tool that can fully expand generated histograms

This tool allows users to sanity check what histograms will be
generated by patterned histograms or histogram suffixes. The difference
between this tool and the print_histogram_names.py is that this tool
also allows you to see what the generated summary and expanded owners.

Bug: 1143748
Change-Id: I55bf2d3c127ecf6292bac08661ede2045a4eaaae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533320
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826608}

tools/metrics/histograms/README.md

@@ -670,6 +670,15 @@ owners:
 As [with histogram entries](#Cleaning-Up-Histogram-Entries), never delete
 variants. If the variant expansion is no longer used, mark it as `<obsolete>`.
 
+*** promo
+Tip: You can run `print_expanded_histogram.py --pattern=` to show all generated
+histograms by patterned histograms or histogram suffixes including their
+summaries and owners. For example, this can be run (from the repo root) as:
+```
+./tools/metrics/histograms/print_expanded_histogram.py --pattern=^UMA.A.B
+```
+***
+
 *** promo
 Tip: You can run `print_histogram_names.py --diff` to enumerate all the
 histogram names that are generated by a particular CL. For example, this can be

tools/metrics/histograms/print_expanded_histograms.py

+#!/usr/bin/env python
+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""Prints expanded histograms."""
+
+import argparse
+import re
+import xml.dom.minidom
+
+import extract_histograms
+import histogram_paths
+import histogram_configuration_model
+import merge_xml
+
+
+def ConstructHistogram(doc, name, histogram_dict):
+  """Constructs a histogram node based on the |histogram_dict|."""
+  histogram = doc.createElement('histogram')
+  # Set histogram node attributes.
+  histogram.setAttribute('name', name)
+  if 'enum' in histogram_dict:
+    histogram.setAttribute('enum', histogram_dict['enum']['name'])
+  else:
+    histogram.setAttribute('units', histogram_dict['units'])
+  if 'expires_after' in histogram_dict:
+    histogram.setAttribute('expires_after', histogram_dict['expires_after'])
+  if histogram_dict.get('base', False):
+    histogram.setAttribute('base', 'True')
+  # Populate the obsolete node.
+  if 'obsolete' in histogram_dict:
+    obsolete_node = doc.createElement('obsolete')
+    obsolete_node.appendChild(doc.createTextNode(histogram_dict['obsolete']))
+    histogram.appendChild(obsolete_node)
+  # Populate owner nodes.
+  for owner in histogram_dict['owners']:
+    owner_node = doc.createElement('owner')
+    owner_node.appendChild(doc.createTextNode(owner))
+    histogram.appendChild(owner_node)
+  # Populate the summary nodes.
+  if 'summary' in histogram_dict:
+    summary_node = doc.createElement('summary')
+    summary_node.appendChild(doc.createTextNode(histogram_dict['summary']))
+    histogram.appendChild(summary_node)
+  return histogram
+
+
+def main(args):
+  # Extract all histograms into a dict.
+  doc = merge_xml.MergeFiles(filenames=histogram_paths.ALL_XMLS,
+                             should_expand_owners=True)
+  histograms, had_errors = extract_histograms.ExtractHistogramsFromDom(doc)
+  if had_errors:
+    raise ValueError("Error parsing inputs.")
+  # Construct a dom tree that is similar to the normal histograms.xml so that
+  # we can use histogram_configuration_model to pretty print it.
+  doc = xml.dom.minidom.Document()
+  configuration = doc.createElement('histogram-configuration')
+  histograms_node = doc.createElement('histograms')
+  for name, histogram in histograms.items():
+    if re.match(args.pattern, name):
+      histograms_node.appendChild(ConstructHistogram(doc, name, histogram))
+  configuration.appendChild(histograms_node)
+  doc.appendChild(configuration)
+  print(histogram_configuration_model.PrettifyTree(doc))
+
+
+if __name__ == '__main__':
+  parser = argparse.ArgumentParser(description='Print expanded histograms.')
+  parser.add_argument('--pattern',
+                      type=str,
+                      default='*',
+                      help='The histogram regex you want to print.')
+  args = parser.parse_args()
+  main(args)