Telemetry: dump app.extensions and app.config.html_theme (#117)

* Telemetry: dump `app.extensions` and `app.config.html_theme`

Dump data from Sphinx into a JSON file that will be read and processed by our
Telemetry collector. This will help us to get some insights about extensions and
theme usage.

* Telemetry: handle unexpected exceptions

* Telemetry: comment about config-inited

* Style

* Telemetry: save only extensions defined by the user

Instead of saving `app.extensions` (all extensions used by Sphinx, including
default ones), we only save `app.config.extensions` which are the ones defined
by the user itself.

readthedocs_ext/readthedocs.py

@@ -338,6 +338,29 @@ def dump_sphinx_data(app, exception):
         )
 
 
+def dump_telemetry(app, config):
+    # We need to get the output directory where the docs are built
+    # _build/json.
+    build_json = os.path.abspath(
+        os.path.join(app.outdir, '..', 'json')
+    )
+    outjson = os.path.join(build_json, 'telemetry.json')
+    outdir = os.path.dirname(outjson)
+
+    try:
+        if not os.path.exists(outdir):
+            os.makedirs(outdir)
+
+        with open(outjson, 'w+') as json_file:
+            data = {
+                'extensions': app.config.extensions,
+                'html_theme': app.config.html_theme,
+            }
+            json.dump(data, json_file, indent=4)
+    except Exception:
+        log.exception("Something went wrong when dumping Telemetry data.")
+
+
 class ReadtheDocsBuilder(StandaloneHTMLBuilder):
 
     """
@@ -396,6 +419,10 @@ def setup(app):
     app.connect('build-finished', remove_search_init)
     app.connect('build-finished', dump_sphinx_data)
 
+    if sphinx.version_info >= (1, 8, 0):
+        # `config-inited` event was introduced in Sphinx 1.8
+        app.connect('config-inited', dump_telemetry)
+
     # Embed
     app.add_directive('readthedocs-embed', EmbedDirective)
     app.add_config_value('readthedocs_embed_project', '', 'html')