Report recent (non-stale) total_difficulty metric
Report recent (non-stale) total_difficulty metric

... by disabling exometer cache for such metric (see upstream issue
105 wrt timer race).

The impact is that exometer_report is less efficient when getting the
total_difficulty metric-datapoint value (once every 10s - see
`exometer_report:add_reporter` in `aec_metrics`).

The loss in efficiency is that exometer_report does not find the entry
in exometer_cache ETS table (of local scheduler ID) hence needs to
call probe process (`exometer_probe:get_value`), that in turns looks
up its local state (`aec_chain_metrics_probe`) and returns last
sample.  (The actual retrieval/sampling of total_difficulty from
aec_conductor process is done asynchronously already - see
`aec_chain_metrics_probe`.)

aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch