Metrics: never count the dashboard's metrics probe as inbound traffic (#149)

On prod (served at root) the dashboard first probes /thermograph/api/v2/metrics before
falling back to root; that 404 was classified as inbound 'other' and shown as an error —
the monitor polluting the traffic it displays. Two fixes: classify any */api/v2/metrics
path as 'metrics' (excluded) whatever prefix it arrives under; and cache the working
metrics URL in the dashboard so it stops re-probing the failing base every refresh.

metrics.py

@@ -98,6 +98,11 @@ def classify_inbound(path: str, base: str = "") -> str:
     the app's mount prefix (e.g. ``/thermograph`` or ``""``).
     """
     p = path or "/"
+    # The dashboard polls the metrics endpoint; never count it as traffic, whatever base
+    # prefix it arrives under — e.g. a `/thermograph/api/v2/metrics` probe against a
+    # root-served prod app would otherwise land in "other" and show as an inbound error.
+    if p.rstrip("/").endswith("/api/v2/metrics"):
+        return "metrics"
     if base and p.startswith(base):
         p = p[len(base):] or "/"
     if p.startswith("/api/"):

tests/test_metrics.py

@@ -34,6 +34,10 @@ def test_classify_inbound_categories():
     assert c("/thermograph/api/v2/notifications", "/thermograph") == "accounts"
     assert c("/thermograph/api/v2/push/subscribe", "/thermograph") == "accounts"
     assert c("/thermograph/api/v2/metrics", "/thermograph") == "metrics"
+    # The metrics endpoint is never counted, even under an unexpected prefix — e.g. the
+    # dashboard probing /thermograph/... against a root-served (base="") prod app.
+    assert c("/thermograph/api/v2/metrics", "") == "metrics"
+    assert c("/api/v2/metrics", "") == "metrics"
     # pages, static, seo
     assert c("/thermograph/", "/thermograph") == "page"
     assert c("/thermograph/calendar", "/thermograph") == "page"