Auth: Add anonymous authn client (#59637)

* Authn: Add Client interface and Reqeust and Identity structures

* Authn: Implement Authenticate method in service

* Authn: Add tracing

* Authn: Add logger

* AuthN: Implement Anonymous client

pkg/services/authn/authnimpl/service.go

@@ -1,8 +1,62 @@
 package authnimpl
 
-import "github.com/grafana/grafana/pkg/services/authn"
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/infra/tracing"
+	"github.com/grafana/grafana/pkg/services/authn"
+	"github.com/grafana/grafana/pkg/services/authn/clients"
+	"github.com/grafana/grafana/pkg/services/org"
+	"github.com/grafana/grafana/pkg/setting"
+	"go.opentelemetry.io/otel/attribute"
+)
 
 var _ authn.Service = new(Service)
 
+func ProvideService(cfg *setting.Cfg, tracer tracing.Tracer, orgService org.Service) *Service {
+	s := &Service{
+		log:     log.New("authn.service"),
+		cfg:     cfg,
+		clients: make(map[string]authn.Client),
+		tracer:  tracer,
+	}
+
+	if s.cfg.AnonymousEnabled {
+		s.clients[authn.ClientAnonymous] = clients.ProvideAnonymous(cfg, orgService)
+	}
+
+	return s
+}
+
 type Service struct {
+	log     log.Logger
+	cfg     *setting.Cfg
+	clients map[string]authn.Client
+
+	tracer tracing.Tracer
+}
+
+func (s *Service) Authenticate(ctx context.Context, clientName string, r *authn.Request) (*authn.Identity, error) {
+	ctx, span := s.tracer.Start(ctx, "authn.Authenticate")
+	defer span.End()
+
+	span.SetAttributes("authn.client", clientName, attribute.Key("authn.client").String(clientName))
+
+	client, ok := s.clients[clientName]
+	if !ok {
+		s.log.FromContext(ctx).Warn("auth client not found", "client", clientName)
+		span.AddEvents([]string{"message"}, []tracing.EventValue{{Str: "auth client is not configured"}})
+		return nil, authn.ErrClientNotFound
+	}
+
+	// FIXME: We want to perform common authentication operations here.
+	// We will add them as we start to implement clients that requires them.
+	// Those operations can be Syncing user, syncing teams, create a session etc.
+	// We would need to check what operations a client support and also if they are requested
+	// because for e.g. basic auth we want to create a session if the call is coming from the
+	// login handler, but if we want to perform basic auth during a request (called from contexthandler) we don't
+	// want a session to be created.
+
+	return client.Authenticate(ctx, r)
 }