Stats: Append scrape duration to the end of `/metrics` response

[Geod24]

Although https://dlang.org/phobos/std_datetime_stopwatch.html#StopWatch might be more suited

[omerfirmak]

Although https://dlang.org/phobos/std_datetime_stopwatch.html#StopWatch might be more suited

this was what docs suggested

// If stopping or resetting the StopWatch is not required, then
// MonoTime can easily be used by itself without StopWatch.
auto before = MonoTime.currTime;
// do stuff...
auto timeElapsed = MonoTime.currTime - before;

[codecov[bot]]

Codecov Report

Merging #3248 (1408b30) into v0.x.x (d360fd7) will decrease coverage by 1.13%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v0.x.x    #3248      +/-   ##
==========================================
- Coverage   54.70%   53.57%   -1.14%     
==========================================
  Files         332      332              
  Lines       35811    35820       +9     
==========================================
- Hits        19592    19189     -403     
- Misses      16219    16631     +412     

Flag	Coverage Δ
integration	23.74% <0.00%> (-6.40%)	:arrow_down:
unittests	87.66% <100.00%> (+0.10%)	:arrow_up:

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

Impacted Files	Coverage Δ
source/agora/node/Runner.d	50.32% <ø> (-5.23%)	:arrow_down:
source/agora/stats/CollectorRegistry.d	96.77% <100.00%> (+0.10%)	:arrow_up:
source/agora/stats/Stats.d	100.00% <100.00%> (ø)
...modules/vibe-core/source/vibe/internal/allocator.d	37.50% <0.00%> (-62.50%)	:arrow_down:
agora/submodules/vibe.d/http/vibe/http/client.d	0.24% <0.00%> (-52.78%)	:arrow_down:
source/agora/network/VibeManager.d	10.00% <0.00%> (-35.72%)	:arrow_down:
agora/submodules/vibe.d/web/vibe/web/rest.d	16.43% <0.00%> (-34.25%)	:arrow_down:
...ra/submodules/vibe.d/stream/vibe/stream/counting.d	28.94% <0.00%> (-13.16%)	:arrow_down:
agora/submodules/vibe.d/data/vibe/data/json.d	5.78% <0.00%> (-11.93%)	:arrow_down:
agora/submodules/base32/src/base32.d	0.00% <0.00%> (-11.12%)	:arrow_down:
... and 28 more

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[Geod24]

Looking at this again, I have to question the motive. Do you expect scrape to take long ? I think that the delays you might be seeing is in Vibe.d handling the request, which means by the time we get to MonoTime.start, most of the delay has already passed.

[omerfirmak]

@Geod24, yeap I suspected the DB operations in /metrics to block the process for a long time occasionally. But havent seen anything proving that.