fix clippy

[jiacai2050]

Meet this error when doing apache/horaedb#641

error[E0658]: `let` expressions in this position are unstable
   --> /Users/jiacai/.cargo/git/checkouts/ceresdb-client-rs-4f2f7fd68ddd1ea7/5fbd1a1/src/db_client/route_based.rs:162:20
    |
162 |                 if let Err(Error::Server(server_error)) = &result &&
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable

For more information about this error, try `rustc --explain E0658`.
error: could not compile `ceresdb-client-rs` due to previous error
warning: build failed, waiting for other jobs to finish...

[Rachelint]

LGTM