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.