chore(ddtrace/otel): document metrics support in rust, php, go

[mabdinur]

What does this PR do? What is the motivation?

Restructures Rust OpenTelemetry API support documentation to follow consistent multi-language directory structure:

• Rust: Creates rust/ directory with metrics.md (private, v0.4.0) and traces.md, removes old rust.md
• Structure: Rust now follows same directory-based organization as Python, .NET, Node.js, and Ruby

Motivation: Standardize Rust documentation structure to match other supported languages and prepare for upcoming Rust metrics support in v0.3.0.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

• Rust metrics tab is hidden (private: true) until v0.4.0 release
• Documentation based strictly on existing dd-trace-rs implementation
• No new content added, only restructuring and formatting
• Go and PHP restructuring will follow in separate commits

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/munir/opentelemetry-metrics-docs-2/opentelemetry/instrument/api_support/rust/
• https://docs-staging.datadoghq.com/munir/opentelemetry-metrics-docs-2/opentelemetry/instrument/api_support/rust/metrics
• https://docs-staging.datadoghq.com/munir/opentelemetry-metrics-docs-2/opentelemetry/instrument/api_support/rust/traces

Removed or renamed files (these should redirect)

• https://docs-staging.datadoghq.com/munir/opentelemetry-metrics-docs-2/opentelemetry/instrument/api_support/rust