Refactor `Middleware` to `Logger` (#845)

* Rename Middleware to Metrics

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Change documentation to reflect the `Metrics` trait

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Middleware name refactoring

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Http middleware renaming

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Rename examples

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update http-server/src/server.rs

Co-authored-by: James Wilson <james@jsdw.me>

* http: Use `request` instead of `HeaderMap` for logging

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Rename `Metrics` -> `Logger`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix cargo clippy

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix examples

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix tests

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix cargo fmt

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix hyper dependency

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

Co-authored-by: James Wilson <james@jsdw.me>
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