Authentication metrics (#1595)

Expose metrics about:

- User login attempts
- Failed user logins
- User logouts
- General successful accesses to SCM-Manager via any authentication realm
- General failed accesses to SCM-Manager

Co-authored-by: Sebastian Sdorra <sebastian.sdorra@cloudogu.com>

scm-webapp/src/main/java/sonia/scm/security/AnonymousRealm.java

@@ -43,15 +43,9 @@ import static com.google.common.base.Preconditions.checkArgument;
 @Extension
 public class AnonymousRealm extends AuthenticatingRealm {
 
-  /**
-   * realm name
-   */
   @VisibleForTesting
   static final String REALM = "AnonymousRealm";
 
-  /**
-   * dao realm helper
-   */
   private final DAORealmHelper helper;
   private final UserDAO userDAO;