feat(metrics): derive with dynamic scope

[rkrasiuk]

closes #782

Provide additional dynamic kv entry for Metrics derive macro. If dynamic = true, the macro will generate fns for dynamic scope instead of operating on a preset one.

Examples

#[derive(Metrics)]
#[metrics(scope = "some_scope")]
struct StaticScopeMetrics {
     // fields...
}

// Describe
StaticScopeMetrics::describe();
// Instantiate
let metrics = StaticScopeMetrics::default();

#[derive(Metrics)]
#[metrics(dynamic = true)]
struct DynamicScopeMetrics {
     // fields...
}

let local_scope = "some_scope";
// Describe
StaticScopeMetrics::describe(local_scope);
// Instantiate
let metrics = DynamicScopeMetrics::new(local_scope);

NOTE: still need to update the docs.

[akirillo]

Ok, this is making sense to me at a high level, and LGTM as it's passing tests!

A couple non-blocking questions from a Rust newbie:

1. It feels like there's a lot of duplicate code between the MetricsScope::Static and MetricsScope::Dynamic branches, the only differences being how metric_name is constructed and what the final quote! block is that's returned - I'm curious, why not just match against the MetricScope enum when defining these two variables, instead of around the entire iteration over metric_fields? I guess the way you do it here avoids branching within each iteration, which is nice.
2. In line with this, why move the generation of metric_name outside of the impl Metric block in metric.rs, and into derive in expand.rs? Doesn't change the API so it doesn't matter much, I guess

[gakonst]

@joshieDo wanna take this over the line once you have some bandwidth from the proptests? @rkrasiuk is sprinting on the new downloader in #881

[rkrasiuk]

@akirillo expressed interest in taking it over as well

[rkrasiuk]

we simply need to fix some docs & comments here, that's it

[akirillo]

yup, will take over on this

[gakonst]

Ah ACK!

[akirillo]

@rkrasiuk added a quick usage example to lib.rs, any other areas of documentation I should touch on? Couldn't see anything else at a glance

[codecov-commenter]

Codecov Report

Merging #785 (f46990f) into main (a1c8a34) will decrease coverage by 0.00%.
The diff coverage is 62.39%.

@@            Coverage Diff             @@
##             main     #785      +/-   ##
==========================================
- Coverage   73.45%   73.44%   -0.01%     
==========================================
  Files         280      280              
  Lines       29697    29856     +159     
==========================================
+ Hits        21813    21928     +115     
- Misses       7884     7928      +44     

Flag	Coverage Δ
unit-tests	73.44% <62.39%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
crates/metrics/metrics-derive/src/lib.rs	100.00% <ø> (ø)
crates/metrics/metrics-derive/src/expand.rs	80.59% <61.06%> (-18.78%)	⬇️
crates/metrics/metrics-derive/src/metric.rs	78.12% <100.00%> (-2.37%)	⬇️
crates/consensus/src/config.rs	50.00% <0.00%> (-1.29%)	⬇️
crates/net/network/src/manager.rs	49.05% <0.00%> (-0.22%)	⬇️
crates/net/network/src/peers/manager.rs	82.55% <0.00%> (-0.19%)	⬇️
crates/executor/src/config.rs	94.87% <0.00%> (+0.04%)	⬆️
crates/executor/src/executor.rs	90.63% <0.00%> (+2.40%)	⬆️

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