Upgrade procfs from 0.12 to 0.13

In addition to upgrading to 0.13, this also switches to a more
efficient way to get the number of open file descriptors.

Author: eminence

Cargo.lock

@@ -105,7 +105,7 @@ features = ["with-chrono-0_4", "with-serde_json-1"]
 
 [target.'cfg(target_os = "linux")'.dependencies]
 # Process information
-procfs = "0.12.0"
+procfs = "0.13.0"
 
 [dev-dependencies]
 criterion = "0.3"

Cargo.toml

@@ -179,7 +179,7 @@ impl Metrics {
 
         let process = Process::myself().unwrap();
         self.open_file_descriptors
-            .set(process.fd().unwrap().len() as i64);
+            .set(process.fd_count().unwrap() as i64);
         self.running_threads
             .set(process.stat().unwrap().num_threads as i64);
     }