use seconds as baseunit for duration metrics

[briend]

Trying to follow some suggested conventions:
https://prometheus.io/docs/practices/naming/

Will produce output like:

# HELP teraslice_query_duration_seconds Total time to complete the named query, in seconds.
# TYPE teraslice_query_duration_seconds gauge
teraslice_query_duration_seconds{query_name="info",url="https://ts-xxx/",name="teraslice-xxx"} 0.182
teraslice_query_duration_seconds{query_name="jobs",url="https://ts-xxx/",name="teraslice-xxx"} 0.251
teraslice_query_duration_seconds{query_name="controllers",url="https://ts-xxx/",name="teraslice-xxx"} 0.185
teraslice_query_duration_seconds{query_name="executions",url="https://ts-xxx/",name="teraslice-xxx"} 0.202503852
teraslice_query_duration_seconds{query_name="state",url="https://ts-xxx/",name="teraslice-xxx"} 0.18

the executions metric seems to have a bit more precision, but not sure if it's worth rounding for aesthetics.

closes #5

[godber]

This looks good, I got a little clarification OOB.