Metrics: Change scrape_duration to milliseconds only

[Geod24]

The default encoding appends a string, which Prometheus doesn't like,
and that kills the whole stats. Using a millisecond resolution is good enough.

Tested live

[codecov[bot]]

Codecov Report

Merging #3251 (7cda725) into v0.x.x (13417d6) will decrease coverage by 58.27%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           v0.x.x    #3251       +/-   ##
===========================================
- Coverage   88.56%   30.29%   -58.28%     
===========================================
  Files         166      266      +100     
  Lines       17029    27090    +10061     
===========================================
- Hits        15082     8207     -6875     
- Misses       1947    18883    +16936     

Flag	Coverage Δ
integration	30.29% <ø> (-4.68%)	:arrow_down:
unittests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/stats/CollectorRegistry.d	40.00% <ø> (-56.78%)	:arrow_down:
source/agora/script/Stack.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/flash/Invoice.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/flash/Network.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/flash/Scripts.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/script/Script.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/utils/Backoff.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/script/Opcodes.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/stats/Collector.d	0.00% <0.00%> (-100.00%)	:arrow_down:
source/agora/stats/StatFormatter.d	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 308 more

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