feat: add fanout failure and target health metrics and v0.1.2

[Kemperino]

• Complete failures are now counted correctly, including when fan_request() returns an error.
• Builder and L2 failures are distinguishable via fanout.
• Every target gets a current-health gauge keyed by sanitized host:port.
• We can alert specifically on:- High absolute count of requests where all targets failed. One target continuously reporting 0 for 20 minutes.

Note

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[Dzejkop]

this is inconsistent with the current setup

I think we should move these as fields into the ProxyMetrics struct

[Dzejkop]

also this made me notice that:

1. We're not calling ProxyMetrics::describe anywhere - and we should

As per the Metrics macro docs:

creates a [Default](https://doc.rust-lang.org/stable/core/default/trait.Default.html) implementation for the struct registering all of
the metrics.

so we should just remove ProxyMetrics::new entirely and just call ::default()