gather_design_stats should not overwrite design stats when provided when multiple logs

[proppy]

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.