gather_design_stats should not overwrite design stats when provided when multiple logs

**Describe the bug**
Not sure if this is the intended behavior, but it seems that `gather_design_stats` always overwrite the same design stats entry when provided with multiple log files.

**To Reproduce**
Run `gather_design_stats` with multiple files, notice that only the last of the provided log files is represented in the resulting proto.

**Expected behavior**
Either of the following behavior would be desirable instead:
- an error when providing more than one log files
- an average of all the metrics from the provided designs (assuming they all refer to the same design)
- a flag to support more than one design stats (and write the resulting data to a repeated proto or a https://github.com/google/riegeli file.