Add temporary metric for theme cookie tween

I'm going to make some changes to this tween, so the metric will help me
keep an eye on how often it's triggering, which should make it easier to
be sure the changes are working as expected.

tildes/tildes/metrics.py

@@ -31,6 +31,9 @@ _COUNTERS = {
     "login_failures": Counter("tildes_login_failures_total", "Login Failures"),
     "messages": Counter("tildes_messages_total", "Messages", labelnames=["type"]),
     "registrations": Counter("tildes_registrations_total", "User Registrations"),
+    "theme_cookie_tween_sets": Counter(
+        "tildes_theme_cookie_tween_sets_total", "Theme Cookies Set by Tween"
+    ),
     "topics": Counter("tildes_topics_total", "Topics", labelnames=["type"]),
     "subscriptions": Counter("tildes_subscriptions_total", "Subscriptions"),
     "unsubscriptions": Counter("tildes_unsubscriptions_total", "Unsubscriptions"),

tildes/tildes/tweens.py

@@ -11,6 +11,8 @@ from pyramid.registry import Registry
 from pyramid.request import Request
 from pyramid.response import Response
 
+from tildes.metrics import incr_counter
+
 
 def http_method_tween_factory(handler: Callable, registry: Registry) -> Callable:
     # pylint: disable=unused-argument
@@ -95,6 +97,8 @@ def theme_cookie_tween_factory(handler: Callable, registry: Registry) -> Callabl
                 secure=True,
                 domain="." + request.domain,
             )
+            incr_counter("theme_cookie_tween_sets")
+
         return response
 
     return theme_cookie_tween