Fix the snapshot in a request counter

Your service counts requests per route in memory, and a dashboard reads snapshots of the counts. In the Go version, Snapshot() returned the live map, so the dashboard read it while request handlers were still writing to it. A snapshot must be a copy of the counts at that moment.

An assistant wrote replay_metrics(event_lines), which replays the counter's events in order. It is in the editor. It has a bug. Find it and fix it.

Input and output:

• event_lines: the events in order. incr|key adds 1 to the count for key. snap|label takes a snapshot named label.
• Return the lines of every snapshot, snapshots in event order. Each line is label:key=count.

Rules the tests check:

• Split a line on |. Skip it unless it has exactly 2 parts.
• Skip an incr with an empty key. Skip any operation other than incr and snap.
• A snapshot holds the counts at the moment it is taken. Later increments do not change it.
• Within a snapshot, list every key counted so far, sorted by plain string comparison (character by character on character codes, so a10 comes before a2).
• A snapshot taken before any increment adds no lines. The label may be empty.