Ensure cache miss logging works in concurrent setting (#2391)

This commit ensures that logging for cache miss should be displayed
properly when multiple threads execute `get_or_load` concurrently.
Specifically, the log should be displayed only once for the first thread
that succeeds in populating a cache value.

Co-authored-by: Yuki Saito <awsaito@amazon.com>

aws/rust-runtime/aws-credential-types/src/cache/lazy_caching.rs

@@ -106,17 +106,23 @@ impl ProvideCachedCredentials for LazyCredentialsCache {
                                 .buffer_time
                                 .mul_f64((self.buffer_time_jitter_fraction)());
 
+                            // Logging for cache miss should be emitted here as opposed to after the call to
+                            // `cache.get_or_load` above. In the case of multiple threads concurrently executing
+                            // `cache.get_or_load`, logging inside `cache.get_or_load` ensures that it is emitted
+                            // only once for the first thread that succeeds in populating a cache value.
+                            info!(
+                                "credentials cache miss occurred; added new AWS credentials (took {:?})",
+                                start_time.elapsed()
+                            );
+
                             Ok((credentials, expiry + jitter))
                         }
                         // Only instrument the the actual load future so that no span
                         // is opened if the cache decides not to execute it.
                         .instrument(span)
                     })
                     .await;
-                info!(
-                    "credentials cache miss occurred; retrieved new AWS credentials (took {:?})",
-                    start_time.elapsed()
-                );
+                debug!("loaded credentials");
                 result
             }
         })