Add network state to system.interval telemetry (#1959)

* add network state to system.interval telemetry

* use serde_json

* use unwrap_or_default

substrate/core/cli/Cargo.toml

@@ -20,6 +20,7 @@ tokio = "0.1.7"
 futures = "0.1.17"
 fdlimit = "0.1"
 exit-future = "0.1"
+serde_json = "1.0"
 sysinfo = "0.8.0"
 panic-handler = { package = "substrate-panic-handler", path = "../../core/panic-handler" }
 client = { package = "substrate-client", path = "../../core/client" }