Fetch the hostname from env variables when running in EKS

Summary:
## This stack

In prod we always read the hostname from `/etc/fbwhoami`. In AWS we provide a custom, hardcoded [`/etc/fbwhoami`](https://fburl.com/code/4nf1tdqi) that hardcodes the hostname.

That wasn't a problem so far because we only ran once instance, but now we are starting to run multiple.

## This diff

Read the hostname from an environment variable (`HOSTNAME`) that is guaranteed to exist on EKS. To avoid any ambiguity, we will only honor it if we are certain to be running on AWS.

This logic is applicable to OSS as well, and guaranteed to work; so we might as well make it unconditional.

Reviewed By: RajivTS

Differential Revision: D70567737

fbshipit-source-id: b3d8351e60989e9f07adcf685b1587b71eebe4de

shed/hostname/src/lib.rs

@@ -16,6 +16,15 @@ use anyhow::Result;
 
 /// Returns hostname as reported by the system
 pub fn get_hostname() -> Result<String> {
+    if let Ok(aws) = std::env::var("AWS_REGION") {
+        if !aws.is_empty() {
+            if let Ok(hostname) = std::env::var("HOSTNAME") {
+                // we are running in AWS and probably in EKS, we can trust the HOSTNAME env var
+                return Ok(hostname);
+            }
+        }
+    }
+
     #[cfg(not(fbcode_build))]
     {
         Ok(::real_hostname::get()?.to_string_lossy().into_owned())