logging: include pid in create_file_writer (meta-pytorch#892)

Summary:
Pull Request resolved: meta-pytorch#892

`fbcode//monarch/hyperactor_mesh:hyperactor_mesh_proxy_test` is a standalone self bootstrapping program that uses `ProcessAllocator` to do the following:
- the driver creates a proc/process to host a `ProxyActor`
- initialization of the `ProxyActor` on the new proc/process creates a proc/process to host a `TestActor`

so, executing this program creates a 3 level process hierarchy  `driver -> parent -> grandchild` where the `parent` process hosts a single proc/process (rank = 0) with one `ProxyActor` and the `grandchild` a single proc/process (rank = 0) with one `TestActor`.

using this program, i observe that as things stand, logs from the parent and the grandchild (since they share a common rank) are merged in the one file `/tmp/$USER/monarch_log_0.stdout`.

this diff disambiguates proc logs by incorporating the process ID of the mesh owner into the proc's log file name.

so, for example, now there will be logs `monarch_log_3529266_0.stdout` (capturing the logs of the parent proc) and `monarch_log_3530444_0.stdout` (capturing the logs of the grandchild proc).

Reviewed By: highker

Differential Revision: D80349615

fbshipit-source-id: 3c37864fa3d5fe327f1d1e679df583fd5e023475

Author: shayne-fletcher

hyperactor_mesh/src/logging.rs

@@ -383,7 +383,18 @@ fn create_file_writer(
     let (path, filename) = log_file_path(env)?;
     let path = Path::new(&path);
     let mut full_path = PathBuf::from(path);
-    full_path.push(format!("{}_{}.{}", filename, local_rank, suffix));
+
+    // This is the PID of the "owner" of the proc mesh, the proc mesh
+    // this proc "belongs" to. In other words,the PID of the process
+    // that invokes `cmd.spawn()` (where `cmd: &mut
+    // tokio::process::Command`) to start the process that will host
+    // the proc that this file writer relates to.
+    let file_created_by_pid = std::process::id();
+
+    full_path.push(format!(
+        "{}_{}_{}.{}",
+        filename, file_created_by_pid, local_rank, suffix
+    ));
     let file = std::fs::OpenOptions::new()
         .create(true)
         .append(true)