histograms: omit empty runs from /tags route (#2989)

Summary:
Analogous to 2fe5301, but for histograms and distributions.

Test Plan:
Tests updated. Manually confirmed that runs like `text_demo` continue to
appear in the run selectors for the histogram and distributions
dashboards even though they have no relevant data.

wchargin-branch: histograms-omit-empty

Author: wchargin

tensorboard/plugins/distribution/distributions_plugin_test.py

@@ -108,7 +108,7 @@ def test_index(self):
                            self._RUN_WITH_LEGACY_DISTRIBUTION,
                            self._RUN_WITH_DISTRIBUTION])
     self.assertEqual({
-        self._RUN_WITH_SCALARS: {},
+        # _RUN_WITH_SCALARS omitted: No distribution data.
         self._RUN_WITH_LEGACY_DISTRIBUTION: {
             self._LEGACY_DISTRIBUTION_TAG: {
                 'displayName': self._LEGACY_DISTRIBUTION_TAG,

tensorboard/plugins/distribution/http_api.md

@@ -31,8 +31,7 @@ Here is an example:
       }
     }
 
-Note that runs without any histogram tags are included as keys with
-value the empty dictionary.
+Runs without any histogram tags are omitted from the result.
 
 ## `/data/plugin/distributions/distributions?run=foo&tag=bar`
 

tensorboard/plugins/histogram/histograms_plugin.py

@@ -112,7 +112,7 @@ def index_impl(self):
       return result
 
     runs = self._multiplexer.Runs()
-    result = {run: {} for run in runs}
+    result = collections.defaultdict(lambda: {})
 
     mapping = self._multiplexer.PluginRunToTagToContent(metadata.PLUGIN_NAME)
     for (run, tag_to_content) in six.iteritems(mapping):

tensorboard/plugins/histogram/histograms_plugin_test.py

@@ -107,7 +107,7 @@ def test_index(self):
                            self._RUN_WITH_LEGACY_HISTOGRAM,
                            self._RUN_WITH_HISTOGRAM])
     self.assertEqual({
-        self._RUN_WITH_SCALARS: {},
+        # _RUN_WITH_SCALARS omitted: No histogram data.
         self._RUN_WITH_LEGACY_HISTOGRAM: {
             self._LEGACY_HISTOGRAM_TAG: {
                 'displayName': self._LEGACY_HISTOGRAM_TAG,

tensorboard/plugins/histogram/http_api.md

@@ -30,8 +30,7 @@ Here is an example:
       }
     }
 
-Note that runs without any histogram tags are included as keys with
-value the empty dictionary.
+Runs without any histogram tags are omitted from the result.
 
 ## `/data/plugin/histograms/histograms?run=foo&tag=bar`