proxy: meter /upstream requests via metrics middleware (#858)

Wrap /upstream/{upstreamPath...} in the metrics middleware so activity
log entries are recorded for model-dispatched endpoints accessed through
the upstream passthrough.

- Move findModelInPath to shared.FindModelInPath and reuse it in
handleUpstream, the log monitor lookup, and FetchContext.
- Extend FetchContext to resolve the model from /upstream/<model>/...
paths without consuming the request body.
- Add isMetricsRecordPath to limit recording to the model-dispatched
endpoints that produce token usage/timings.
- Add tests for upstream metrics recording and FetchContext upstream
path resolution.

Fixes #855

internal/shared/http.go

@@ -91,16 +91,24 @@ func SendResponse(w http.ResponseWriter, r *http.Request, status int, message st
 	w.Write(resp)
 }
 
-// FetchContext will attempt to get the model id from the context then
-// from the model body. If it extracts the model from the body it will
-// store the model in the context for downstream handlers. An error
-// will be returned when model can not be fetch from either location.
+// FetchContext will attempt to get the model id from the context, then
+// from an /upstream/<model> path prefix, then from the request body/query.
+// If it extracts the model it will store it in the context for downstream
+// handlers. An error will be returned when a model cannot be identified.
 func FetchContext(r *http.Request, cfg config.Config) (ReqContextData, error) {
 	data, ok := ReadContext(r.Context())
 	if ok {
 		return data, nil
 	}
 
+	if strings.HasPrefix(r.URL.Path, "/upstream/") {
+		if data, ok := extractUpstreamContext(r, cfg); ok {
+			*r = *r.WithContext(SetContext(r.Context(), data))
+			return data, nil
+		}
+		return ReqContextData{}, ErrNoModelInContext
+	}
+
 	if data, err := extractContext(r); err == nil && data.Model != "" {
 		realName, _ := cfg.RealModelName(data.Model)
 		if realName == "" {
@@ -117,6 +125,59 @@ func FetchContext(r *http.Request, cfg config.Config) (ReqContextData, error) {
 	return ReqContextData{}, ErrNoModelInContext
 }
 
+// extractUpstreamContext resolves the model from an /upstream/<model>/... path.
+func extractUpstreamContext(r *http.Request, cfg config.Config) (ReqContextData, bool) {
+	searchName, realName, _, found := FindModelInPath(cfg, strings.TrimPrefix(r.URL.Path, "/upstream"))
+	if !found {
+		return ReqContextData{}, false
+	}
+	return ReqContextData{
+		Model:            searchName,
+		ModelID:          realName,
+		ApiKey:           ExtractAPIKey(r),
+		Streaming:        r.URL.Query().Get("stream") == "true",
+		SendLoadingState: sendLoadingState(cfg, realName),
+		Metadata:         make(map[string]string),
+	}, true
+}
+
+// sendLoadingState reports whether the configured model wants loading-state SSEs.
+func sendLoadingState(cfg config.Config, modelID string) bool {
+	if mc, ok := cfg.Models[modelID]; ok {
+		return mc.SendLoadingState != nil && *mc.SendLoadingState
+	}
+	return false
+}
+
+// FindModelInPath walks a slash-separated path, building up segments until one
+// matches a configured model. This resolves model names that contain slashes
+// (e.g. "author/model"). Returns the matched name, its real model ID, the
+// remaining path, and whether a match was found.
+func FindModelInPath(cfg config.Config, path string) (searchName, realName, remainingPath string, found bool) {
+	parts := strings.Split(strings.TrimSpace(path), "/")
+	name := ""
+
+	for i, part := range parts {
+		if part == "" {
+			continue
+		}
+		if name == "" {
+			name = part
+		} else {
+			name = name + "/" + part
+		}
+
+		if modelID, ok := cfg.RealModelName(name); ok {
+			searchName = name
+			realName = modelID
+			remainingPath = "/" + strings.Join(parts[i+1:], "/")
+			found = true
+		}
+	}
+
+	return
+}
+
 func SetContext(ctx context.Context, data ReqContextData) context.Context {
 	return context.WithValue(ctx, ReqContextKey, data)
 }