As suggested in prometheus#64,
this commit update edition key of manifest file into 2021.

Before update, I ran 'cargo fix --edition' to check if migration is
needed as this guide(https://doc.rust-lang.org/edition-guide/rust-2021/prelude.html#migration)
says.
There was no change but following message was printed about the
upstream dependencies as mentioned in (https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html#migration)
'When building the following dependencies, the given features will no longer be used:
  syn v1.0.95 (as host dependency) removed features: extra-traits, full, visit, visit-mut

The following differences only apply when building with dev-dependencies:
  standback v0.2.17 (as host dependency) removed features: std
  syn v1.0.95 (as host dependency) removed features: visit'

Signed-off-by: Doehyun Baek <doehyunbaek@gmail.com>

As documented in 2021 edition guide(https://doc.rust-lang.org/edition-guide/rust-2021/IntoIterator-for-arrays.html),
array.into_iter() now returns owned value instead of references,
This commit changes previous doc comments to utilize new behavior.

Signed-off-by: Doehyun Baek <doehyunbaek@gmail.com>

Accordiing to suggestions here(prometheus#65 (review)).

Signed-off-by: Doehyun Baek <doehyunbaek@gmail.com>

Signed-off-by: Doehyun Baek <doehyunbaek@gmail.com>