Ignore trace stats file in fmt command

The command was failing for snapshot directories containing both trace
and trace stats snapshots because it assumes that all json files are
trace snapshots.

Just ignore the trace stats snapshots for now.

ddapm_test_agent/fmt.py

@@ -81,8 +81,18 @@ def main(args: Optional[List[str]] = None) -> None:
     resolved_files = _resolve_files(parsed_args.files)
     log.info("Found %d snapshot files to process", len(resolved_files))
 
+    trace_files = []
+    trace_stats_files = []
+    for f in resolved_files:
+        if f.endswith("_tracestats.json"):
+            trace_stats_files.append(f)
+        else:
+            trace_files.append(f)
+    log.info("Found %d trace snapshot files to process", len(trace_files))
+    log.info("Found %d trace stats snapshot files to process", len(trace_stats_files))
+
     has_errors = False
-    for fname in resolved_files:
+    for fname in trace_files:
         log.debug("Checking snapshot file %r", fname)
         try:
             # Read the original file data
@@ -107,7 +117,7 @@ def main(args: Optional[List[str]] = None) -> None:
                     log.info("Snapshot file %r was formatted", fname)
 
         except Exception:
-            log.error("Error processing file %r", fname)
+            log.exception("Error processing file %r", fname)
             has_errors = True
 
     if has_errors: