Fix clippy lint on missing #[must_use] attributes

[msrd0]

Description

Fix clippy lint on missing #[must_use] attributes

Checklist

• Formatted code using cargo fmt --all
• Linted code using clippy
  • with reqwest feature: cargo clippy --manifest-path influxdb/Cargo.toml --all-targets --no-default-features --features use-serde,derive,reqwest-client -- -D warnings
  • with surf feature: cargo clippy --manifest-path influxdb/Cargo.toml --all-targets --no-default-features --features use-serde,derive,hyper-client -- -D warnings
• Updated README.md using cargo readme -r influxdb -t ../README.tpl > README.md
• Reviewed the diff. Did you leave any print statements or unnecessary comments?
• Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment