chore(layers): unify layer crates and gate dtrace on linux

[koushiro]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

• add crate-level docs, doc_cfg, and missing_docs lint across layer crates
• mark layers non_exhaustive and hide internal accessors/wrappers
• move dtrace dependency to linux target and tidy Cargo.toml/workspace dependency specs

Are there any user-facing changes?

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[Copilot]

Pull request overview

This PR unifies layer crates across the OpenDAL project by standardizing their structure and documentation.

• Adds comprehensive crate-level documentation, doc_cfg attributes, and missing_docs lints to all layer crates
• Standardizes layer structs by adding non_exhaustive attribute and providing new() constructors
• Marks internal accessor and wrapper types as #[doc(hidden)] to clean up the public API surface

Reviewed changes

Copilot reviewed 34 out of 36 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
core/layers/tracing/src/lib.rs	Added crate docs, marked TracingLayer non_exhaustive, hid internal types
core/layers/timeout/src/lib.rs	Added crate docs, updated examples, removed Clone from accessor
core/layers/timeout/Cargo.toml	Removed unnecessary default-features specification
core/layers/throttle/src/lib.rs	Added crate docs, hid internal types, removed Clone from accessor
core/layers/throttle/Cargo.toml	Removed unnecessary features specification for governor
core/layers/tail-cut/src/lib.rs	Added crate docs, reorganized Default/new methods
core/layers/retry/src/lib.rs	Added crate docs, updated examples to use default(), improved test error handling
core/layers/retry/Cargo.toml	Cleaned up dev-dependency specifications
core/layers/prometheus/src/lib.rs	Added crate docs, hid internal interceptor
core/layers/prometheus/Cargo.toml	Removed default-features from observe-metrics-common
core/layers/prometheus-client/src/lib.rs	Added crate docs, hid internal interceptor
core/layers/prometheus-client/Cargo.toml	Removed default-features from observe-metrics-common
core/layers/oteltrace/src/lib.rs	Added crate docs, marked layer non_exhaustive with new()
core/layers/otelmetrics/src/lib.rs	Added crate docs, hid internal interceptor
core/layers/otelmetrics/Cargo.toml	Removed default-features from observe-metrics-common
core/layers/observe-metrics-common/src/lib.rs	Updated library description, formatted tables, hid internal types
core/layers/observe-metrics-common/Cargo.toml	Simplified futures dependency specification
core/layers/mime-guess/src/lib.rs	Added crate docs, improved tests with Result, removed Clone from accessor
core/layers/metrics/src/lib.rs	Added crate docs, marked layer non_exhaustive, simplified interceptor
core/layers/logging/src/lib.rs	Added crate docs, added Clone to layer, removed from accessor
core/layers/immutable-index/src/lib.rs	Added crate docs, fixed doc example format, removed Clone from accessor
core/layers/hotpath/src/lib.rs	Added crate docs, marked layer non_exhaustive
core/layers/fastrace/src/lib.rs	Added crate docs, marked layer non_exhaustive
core/layers/fastmetrics/src/lib.rs	Added crate docs, hid internal interceptor
core/layers/fastmetrics/Cargo.toml	Removed unnecessary features specification
core/layers/dtrace/src/lib.rs	Added crate docs, gated to Linux-only with cfg, fixed example
core/layers/dtrace/Cargo.toml	Moved dependencies to Linux-specific target configuration
core/layers/concurrent-limit/src/lib.rs	Added crate docs, cleaned up imports, removed Clone from accessor
core/layers/chaos/src/lib.rs	Added crate docs, reorganized imports
core/layers/capability-check/src/lib.rs	Added crate docs, reorganized Debug impl
core/layers/await-tree/src/lib.rs	Added crate docs, removed Clone from accessor
core/layers/async-backtrace/src/lib.rs	Added crate docs, removed Clone from accessor
core/core/src/docs/upgrade.md	Fixed trailing whitespace and formatting
core/core/src/docs/performance/http_optimization.md	Fixed trailing whitespace
core/Cargo.toml	Moved dtrace dependency to Linux-specific target
core/Cargo.lock	Updated with tokio dependency for dtrace

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[Xuanwo]

Thank you for working on this!