feat: add_link and with_link

[andylokandy]

Resolves #148

[Copilot]

Pull request overview

This pull request adds support for span links to the fastrace library, resolving issue #148. Span links allow spans to reference other spans without establishing strict parent-child relationships, which is a standard feature in OpenTelemetry.

Changes:

• Adds with_link() and add_link() methods to both Span and LocalSpan APIs for adding span links
• Deprecates enter_with_parents() method, converting multi-parent functionality to use the first parent with additional parents as links
• Refactors internal architecture: changes CollectToken from Vec<CollectTokenItem> to a single struct, and changes properties field from Option<Vec> to Vec directly

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
fastrace/src/span.rs	Adds with_link and add_link methods, deprecates enter_with_parents, refactors CollectToken usage
fastrace/src/local/local_span.rs	Adds with_link and add_link methods for LocalSpan
fastrace/src/local/raw_span.rs	Adds links field, changes properties to non-optional, derives Clone
fastrace/src/collector/mod.rs	Adds links field to SpanRecord, refactors CollectToken to single struct
fastrace/src/collector/global_collector.rs	Adds Link handling in DanglingItem enum and processing logic
fastrace/src/collector/id.rs	Adds PartialEq and Eq derives to SpanContext for test assertions
fastrace-opentelemetry/src/lib.rs	Adds map_links function to convert fastrace links to OpenTelemetry format
fastrace/src/local/span_queue.rs	Adds add_link and with_link methods
fastrace/src/local/local_span_line.rs	Adds link handling methods, refactors token handling
fastrace/src/local/local_span_stack.rs	Adds link handling methods
fastrace/src/util/tree.rs	Updates to use single CollectToken instead of Vec
fastrace/src/util/mod.rs	Removes CollectToken type alias (moved to collector module)
fastrace/src/event.rs	Changes properties to non-optional, fixes doc example
fastrace/tests/lib.rs	Adds tests for span links, removes deprecated multi-parent test
tests/statically-disable/src/main.rs	Updates test to use new link APIs
examples/synchronous.rs	Updates to use with_properties instead of with_property
examples/asynchronous.rs	Updates to use with_properties instead of with_property

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