Both build and tests pass. Here's a summary of the changes:

**`pkg/metricstore/parquetArchive.go`:**
- Removed `w.writer.Flush()` from `WriteCheckpointFile()` — it no longer creates a row group per call
- Added new `FlushRowGroup()` method that wraps `w.writer.Flush()` for explicit caller control

**`pkg/metricstore/archive.go`:**
- Added `writer.FlushRowGroup()` call after all checkpoint files for a host are written, flushing **once per host** instead of once per checkpoint file

This reduces the maximum row group count from `hosts × checkpoints_per_host` (which exceeded 32767) to just `hosts` (typically ~3000 for large clusters), well within the parquet limit.