AA wants latency measurements for each input collection / output topic pair. This ticket covers adding the engine stats for this.

This will build on this PR. Below is a rough explanation of what to do:

1. Put these code changes under a feature flag (set to false for now).
2. Confirm the new stats() response with Joe and Laura. Let's assume a schema like below for now:

stats() response

{
  ...
  latency: {
    ... existing fields p50,p99,sum,count,start,end ...,
    targets: [
       {
         input: {connectionName, db, coll},
         output: {connectionName, topic},
         p50, p99, sum, count
       },
       ...
    ]
  }
}

2. Add unordered_map<Target, Summary> perInputOutputLatency in OperatorStats... this is only set by Sink operator (just like overallLatency).

3. Amend ChangeStreamSourceOperator to seta new Target StreamDocument::inputTarget field in StreamDocument.

4. Amend KafkaEmitOperator to add measurements to perInputOutputLatency.