dpd should accumulate data from clear-on-read counters

When accessing some tofino counters, the ASIC clears the values after reading them. This means that historical data will be lost, and multiple admins servicing the same switch will be getting incomplete information.

When dpd reads counters in response to a swadm request, the values should be accumulated in an in-core cache. We can then use the accumulated date to either return the total counter values or just that data generated since the last read.

Start by tracing dpd's swadm counter-read path and the Tofino handling for clear-on-read counters; the issue does not name specific files or tests. Inspect how counter values are represented and whether an in-core cache already exists. Done means repeated reads preserve historical values and can return either accumulated totals or data since the previous read.