Can't run clippy

[nstinus]

metrics-exporter-prometheus has clippy lints enabled. Yet, many are outstanding.

For example:

error: use of `or_insert_with` to construct default value
  --> metrics-exporter-prometheus/src/recorder.rs:89:18
   |
89 |                 .or_insert_with(IndexMap::new)
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `or_default()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_default
note: the lint level is defined here
  --> metrics-exporter-prometheus/src/lib.rs:97:9
   |
97 | #![deny(clippy::all)]
   |         ^^^^^^^^^^^
   = note: `#[deny(clippy::unwrap_or_default)]` implied by `#[deny(clippy::all)]`

error: could not compile `metrics-exporter-prometheus` (lib) due to 6 previous errors

Either:

• downgrade them to warnings or,
• add a check in the ci.

[tobz]

I think this should be all set now that #417 has been merged.