chore: address clippy warnings

[duncanpharvey]

What does this PR do?

Addresses minor clippy warnings.

Motivation

cargo clippy returns no warnings.

Additional Notes

% cargo clippy
    Checking datadog-trace-agent v0.1.0 (/Users/duncan.harvey/go/src/github.com/DataDog/serverless-components/crates/datadog-trace-agent)
warning: you should consider adding a `Default` implementation for `Tags`
  --> crates/datadog-trace-agent/src/config.rs:46:5
   |
46 | /     pub fn new() -> Self {
47 | |         Self {
48 | |             tags: HashMap::new(),
49 | |             function_tags_string: OnceLock::new(),
50 | |         }
51 | |     }
   | |_____^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#new_without_default
   = note: `#[warn(clippy::new_without_default)]` on by default
help: try adding this
   |
26 + impl Default for Tags {
27 +     fn default() -> Self {
28 +         Self::new()
29 +     }
30 + }
   |

warning: this function has too many arguments (8/7)
   --> crates/datadog-trace-agent/src/mini_agent.rs:180:5
    |
180 | /     async fn trace_endpoint_handler(
181 | |         config: Arc<config::Config>,
182 | |         req: hyper_migration::HttpRequest,
183 | |         trace_processor: Arc<dyn trace_processor::TraceProcessor + Send + Sync>,
...   |
188 | |         proxy_tx: Sender<ProxyRequest>,
189 | |     ) -> http::Result<hyper_migration::HttpResponse> {
    | |____________________________________________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#too_many_arguments
    = note: `#[warn(clippy::too_many_arguments)]` on by default

warning: useless use of `format!`
   --> crates/datadog-trace-agent/src/trace_processor.rs:107:18
    |
107 |                 &format!("Error processing traces: Payload too large"),
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `"Error processing traces: Payload too large".to_string()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#useless_format
    = note: `#[warn(clippy::useless_format)]` on by default

warning: `datadog-trace-agent` (lib) generated 3 warnings (run `cargo clippy --fix --lib -p datadog-trace-agent` to apply 2 suggestions)
    Checking datadog-serverless-compat v0.1.0 (/Users/duncan.harvey/go/src/github.com/DataDog/serverless-components/crates/datadog-serverless-compat)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.24s

Describe how to test/QA your changes

Run cargo clippy and verify there are no more warnings.