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

[mabdinur]

What does this PR do? What is the motivation?

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

• Rust: Creates rust/ directory with metrics.md (private, v0.4.0) and traces.md, removes old rust.md
• Go: Creates go/ directory with metrics.md and traces.md, following the same directory-based organization
• PHP: Creates php/ directory with metrics.md and traces.md, following the same directory-based organization

Structure:

• Rust, Go, and PHP now follow the same directory-based organization as Python, .NET, Node.js, and Ruby

Motivation:

• Standardize documentation structure across all supported languages and prepare for upcoming metrics support in Rust (v0.3.0), Go (v2.6.0), and PHP (v1.16.0).

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

[OliviaShoup]

thank you for the PR! just a heads up: because of the size, it'll take a little longer to be reviewed by the docs team

[mabdinur]

thank you for the PR! just a heads up: because of the size, it'll take a little longer to be reviewed by the docs team

No rush on this. This PR depends on #33640, either one can be merged first, but merging one will introduce conflicts that will need to be resolved in the other.