## Problem
The `src/proxy.rs` file is far too large
## Summary of changes
Creates 3 new files:
```
src/metrics.rs
src/proxy/retry.rs
src/proxy/connect_compute.rs
```
## Summary of changes
saw some low-hanging codecov improvements. even if code coverage is
somewhat of a pointless game, might as well add tests where we can and
delete code if it's unused
## Problem
Single rate bucket is limited in usefulness
## Summary of changes
Introduce a secondary bucket allowing an average of 200 requests per
second over 1 minute, and a tertiary bucket allowing an average of 100
requests per second over 10 minutes.
Configured by using a format like
```sh
proxy --endpoint-rps-limit 300@1s --endpoint-rps-limit 100@10s --endpoint-rps-limit 50@1m
```
If the bucket limits are inconsistent, an error is returned on startup
```
$ proxy --endpoint-rps-limit 300@1s --endpoint-rps-limit 10@10s
Error: invalid endpoint RPS limits. 10@10s allows fewer requests per bucket than 300@1s (100 vs 300)
```
## Problem
1. Using chrono for durations only is wasteful
2. The arc/mutex was not being utilised
3. Locking every shard in the dashmap every GC could cause latency
spikes
4. More buckets
## Summary of changes
1. Use `Instant` instead of `NaiveTime`.
2. Remove the `Arc<Mutex<_>>` wrapper, utilising that dashmap entry
returns mut access
3. Clear only a random shard, update gc interval accordingly
4. Multiple buckets can be checked before allowing access
When I benchmarked the check function, it took on average 811ns when
multithreaded over the course of 10 million checks.
