You did an interesting exercise
So, the first line in setup() is:
t0 = millis();
the last line before creating the ThingSpeak data string is
tTot = millis() - t0; // get ms since boot
I then build the data string
data = "field1="+String(oneWireVal[0],1) + "&field2="+String(oneWireVal[1],1) + "&field3="+String(tTot);
BTW, I tried MQTT for a couple of days and reverted back to HTTP
In my opinion, ThingSpeak implementation is not yet ready for prime time
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