changefeedccl: collect stats for distribution of spans along various dimensions

As we implement the better span-level checkpointing project, we've identified that there's an observability gap in the distribution of tracked spans along various dimensions (e.g. resolved timestamp, throughput). We should create histograms for each dimension and output them in structured logs. Once we have more data, we can think about adding heuristics to improve span selection for span-level checkpoints.

Jira issue: CRDB-46765