Macro register_extra_span_field 

macro_rules! register_extra_span_field {
    ($key:expr) => { ... };
}

Register a tracing-span field name that downstream crates want preserved on Vector’s internal observability output.

A single registration covers both output channels:

• On metrics, the field is added to the allowlist consulted by VectorLabelFilter (alongside Vector’s built-in component_id, component_type, component_kind, buffer_type), so metrics-tracing-context no longer drops it before the metrics registry sees it.
• On logs/traces emitted via internal_logs, the field is added to the allowlist consulted by SpanFields (alongside the existing component_* prefix gate), so it is captured onto the log event under vector.<field>.

Example: an embedder that owns a “deployment-version” concept of its own can write register_extra_span_field!("deployment_version"); once at module scope and any internal metric or log emitted from inside a span carrying that field will inherit it.

Registrations are collected at link time via the inventory crate, so both read paths are lock-free. The expansion goes through this crate’s re-exports of inventory, MetricLabel, and SpanField, so callers do not need a direct inventory dependency.