ftr: ExponentiallyDecayingReservoir support

[flycash]

What this PR does:
Implementing ExponentiallyDecayingReservoir, which uses forward, exponentially, priority algorithm. And it will be the default Reservoir implementation
Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #319 into feature/metrics will increase coverage by 1.21%.
The diff coverage is 90.81%.

@@                 Coverage Diff                 @@
##           feature/metrics     #319      +/-   ##
===================================================
+ Coverage             67.4%   68.61%   +1.21%     
===================================================
  Files                  133      135       +2     
  Lines                 7908     8090     +182     
===================================================
+ Hits                  5330     5551     +221     
+ Misses                2098     2053      -45     
- Partials               480      486       +6

Impacted Files	Coverage Δ
config_center/configurator/override.go	75.4% <ø> (ø)	⬆️
cluster/router/condition_router.go	78.35% <ø> (ø)	⬆️
common/url.go	69.28% <ø> (ø)	⬆️
registry/protocol/protocol.go	81.73% <ø> (ø)	⬆️
config/graceful_shutdown.go	58.62% <ø> (ø)	⬆️
metrics/impl/default_compass.go	87.7% <25%> (-1.56%)	⬇️
metrics/impl/weighted_snapshot.go	92.15% <92.15%> (ø)
metrics/impl/exponentially_decaying_reservoir.go	92.4% <92.4%> (ø)
cluster/cluster_impl/failback_cluster_invoker.go	78.49% <0%> (-2.16%)	⬇️
... and 7 more

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