Bring back the process feature of prometheus

This is extremely useful, in particular to watch the number of
file descriptors used by the application.

Unfortunately, the prometheus crate is currently broken on 32-bit
platforms.

For that reason, Until https://github.com/tikv/rust-prometheus/pull/446
is merged, we have to use a fork on that crate.

Cargo.toml

@@ -62,8 +62,10 @@ privdrop = "0.5.2"
 
 [dependencies.prometheus]
 optional = true
+package = "prometheus-32bitfix"
 version = "0.13.1"
 default_features = false
+features = ["process"]
 
 [features]
 default = ["metrics"]