remote_storage/abs: count 404 and 304 for get as ok for metrics (#9912)

## Problem

We currently see elevated levels of errors for GetBlob requests. This is
because 404 and 304 are counted as errors for metric reporting.

## Summary of Changes

Bring the implementation in line with the S3 client and treat 404 and
304 responses as ok for metric purposes.

Related: neondatabase/cloud#20666

libs/remote_storage/src/azure_blob.rs

@@ -220,6 +220,11 @@ impl AzureBlobStorage {
         let started_at = ScopeGuard::into_inner(started_at);
         let outcome = match &download {
             Ok(_) => AttemptOutcome::Ok,
+            // At this level in the stack 404 and 304 responses do not indicate an error.
+            // There's expected cases when a blob may not exist or hasn't been modified since
+            // the last get (e.g. probing for timeline indices and heatmap downloads).
+            // Callers should handle errors if they are unexpected.
+            Err(DownloadError::NotFound | DownloadError::Unmodified) => AttemptOutcome::Ok,
             Err(_) => AttemptOutcome::Err,
         };
         crate::metrics::BUCKET_METRICS