decrease default tracing permits

[mattsse]

Describe the feature

The default tracing permits:

reth/crates/rpc/rpc/src/debug.rs

Line 695 in 422b8f8

let _permit = self.acquire_trace_permit().await;

are too high. tracing is mostly cpu bound, so the default should be max(std::thread::available_parallelism() - x, 2) where x could be 2-4.
This way we don't allow saturating all cores with tracing requests, risking starvation of essential jobs

reth/crates/node-core/src/args/rpc_server_args.rs

Lines 150 to 152 in 422b8f8

	/// Maximum number of concurrent tracing requests.
	#[arg(long, value_name = "COUNT", default_value_t = constants::DEFAULT_MAX_TRACING_REQUESTS)]
	pub rpc_max_tracing_requests: u32,

reth/crates/rpc/rpc-builder/src/constants.rs

Lines 22 to 23 in 422b8f8

	/// The default maximum number of concurrently executed tracing calls
	pub const DEFAULT_MAX_TRACING_REQUESTS: u32 = 25;

Additional context

No response

[AbnerZheng]

Can I give it a try?