*: Rename prometheus-exporter crate to substrate-prometheus-end… (#5076)

This patch renames the crate for the following two reasons:

1. The prometheus-exporter crate introduces native in-process Prometheus
style instrumentation to the Substrate project. Within the Prometheus
ecosystem the term "exporter" is used for external processes exposing
metrics for e.g. the Linux Kernel. In-process exposition would be
described via the term "endpoint".

2. "prometheus-exporter" is generic and ignores the fact that it is only
usable within the context of Substrate. In addition the name
"prometheus-exporter" is already taken on crates.io.

substrate/utils/prometheus/src/lib.rs

@@ -65,7 +65,7 @@ mod known_os {
 		Http(hyper::http::Error),
 		/// i/o error.
 		Io(std::io::Error),
-		#[display(fmt = "Prometheus exporter port {} already in use.", _0)]
+		#[display(fmt = "Prometheus port {} already in use.", _0)]
 		PortInUse(SocketAddr)
 	}