Switch the billing metrics storage format to ndjson. (#12427)

## Problem
The billing team wants to change the billing events pipeline and use a
common events format in S3 buckets across different event producers.

## Summary of changes
Change the events storage format for billing events from JSON to NDJSON.
Also partition files by hours, rather than days.

Resolves: https://github.com/neondatabase/cloud/issues/29995

test_runner/regress/test_pageserver_metric_collection.py

@@ -180,7 +180,7 @@ def test_metric_collection(
     httpserver.check()
 
     # Check that at least one bucket output object is present, and that all
-    # can be decompressed and decoded.
+    # can be decompressed and decoded as NDJSON.
     bucket_dumps = {}
     assert isinstance(env.pageserver_remote_storage, LocalFsStorage)
     for dirpath, _dirs, files in os.walk(env.pageserver_remote_storage.root):
@@ -188,7 +188,13 @@ def test_metric_collection(
             file_path = os.path.join(dirpath, file)
             log.info(file_path)
             if file.endswith(".gz"):
-                bucket_dumps[file_path] = json.load(gzip.open(file_path))
+                events = []
+                with gzip.open(file_path, "rt") as f:
+                    for line in f:
+                        line = line.strip()
+                        if line:
+                            events.append(json.loads(line))
+                bucket_dumps[file_path] = {"events": events}
 
     assert len(bucket_dumps) >= 1
     assert all("events" in data for data in bucket_dumps.values())