Misc. dependency cleanup (#6810)

* remove ensure_dir_exists (2 deps saved)

* group UNHANDLED_ERRORs into a generic (2 deps saved)

* Introduce separate `health_metrics` crate

* separate health_metrics crate

* remove metrics from warp_utils

* move ProcessHealth::observe and SystemHealth::observe to health_metrics

* fix errors

* nitpick `Cargo.toml`s

---------

Co-authored-by: Daniel Knopik <daniel@dknopik.de>
# Conflicts:
#	Cargo.toml

validator_client/http_api/src/lib.rs

@@ -32,6 +32,7 @@ use eth2::lighthouse_vc::{
         PublicKeyBytes, SetGraffitiRequest,
     },
 };
+use health_metrics::observe::Observe;
 use lighthouse_version::version_with_platform;
 use logging::SSELoggingComponents;
 use parking_lot::RwLock;