cmd/geth, rpc/api: extend metrics API, add a basic monitor command

cmd/geth/main.go

@@ -214,6 +214,16 @@ The Geth console is an interactive shell for the JavaScript runtime environment
 which exposes a node admin interface as well as the Ðapp JavaScript API.
 See https://github.com/ethereum/go-ethereum/wiki/Javascipt-Console.
 This command allows to open a console on a running geth node.
+`,
+		},
+		{
+			Action: monitor,
+			Name:   "monitor",
+			Usage:  `Geth Monitor: node metrics monitoring and visualization`,
+			Description: `
+The Geth monitor is a tool to collect and visualize various internal metrics
+gathered by the node, supporting different chart types as well as the capacity
+to display multiple metrics simultaneously.
 `,
 		},
 		{