cells: allow configuration selection for JFR

Motivation:
Java flight recorder starts according to a specific configuration, which
can be either one of the pre-defined options provided by JVM or user
supplied configuration.

Modification:
Update `jfr start` admin command to accept ether pre-defined
configuration name ('default' or 'profile') or a path to customized
configuration file.

Result:
More flexible out-of-box profiling.

Acked-by: Albert Rossi
Target: master
Require-book: no
Require-notes: yes

modules/cells/src/main/java/dmg/cells/nucleus/SystemCell.java

@@ -5,6 +5,7 @@
 import com.google.common.base.Throwables;
 import com.google.common.util.concurrent.MoreExecutors;
 import dmg.util.AuthorizedString;
+import dmg.util.command.Argument;
 import dmg.util.command.Command;
 import dmg.util.command.Option;
 import dmg.util.logback.FilterShell;
@@ -146,22 +147,30 @@ public String call() {
     }
 
 
-    @Command(name = "jfr start", hint = "Starts Java flight recorder", description = "Starts JFR")
+    @Command(name = "jfr start", hint = "Starts Java flight recorder.",
+          description = "Starts Java flight recorder. The JFR configuration can be specified as optional"
+                + " parameter: either as one of JVM pre-defined configuration names 'default' or 'profile', or"
+                + " as an absolute path to a custom configuration."
+    )
     public class StartJFR implements Callable<String> {
 
+        @Argument(usage = "Predefined JFR configuration.", metaVar = "configuration", required = false, valueSpec = "[default|profile|<path>]")
+        String config = "default";
+
         @Override
         public String call() throws Exception {
 
             if (recording != null) {
                 return "Another record in progress.";
             }
 
-            Configuration configuration = Configuration.getConfiguration("default");
+            var configuration = config.startsWith("/") ?
+                  Configuration.create(Path.of(config)) : Configuration.getConfiguration(config);
             recording = new Recording(configuration);
             recording.setName(getCellDomainName());
             recording.start();
 
-            return "enabled";
+            return "enabled with config: " + configuration.getName();
         }
     }