zephyr: coap: periodically report resend count

Sum the number of coap request resends over a period of time and report as an INF log message. Change the existing resend message to DBG level so it is still available when that log level is selected via Kconfig.

I'm proposing this approach as an alternative to #500. We had originally discussed changing the log level of zephyr_coap_req.c in only the golioth log backend so the messages are reported in the terminal but not sent to the cloud. This has two disadvantages:

Setting a runtime level for that backend felt a little bit convoluted as it needs to be done after the backend is initialized and we don't initialize until after we connect to Golioth
When only a few resend messsages occur, they're pretty handy to have (even on remote logs).

This PR is a rather small resource change (adds 6 bytes to the Golioth client) and low overhead (we already have the uptime value so it's just a some subtraction/multiplication on each conditional).

resolves https://github.com/golioth/firmware-issue-tracker/issues/576 closes #500

golioth / golioth-firmware-sdk

zephyr: coap: periodically report resend count #516