RequestHeaderProvider

Supplies extra request headers computed fresh for each attempt — the hook for trace-context propagation headers (W3C traceparent/tracestate, baggage, or an equivalent from any other tracing system) derived from whatever context is active right now.

Unlike NetworkEndpoint.headers, which is a fixed value on the endpoint instance, headersFor is called immediately before each attempt is sent, so it can reflect state set up for that specific attempt — e.g. a span started by a RequestInterceptor around the same attempt index, whose tracing library now considers that span "current".

Headers returned here are applied after NetworkEndpoint.headers, so they augment rather than remove endpoint-defined headers; on a name collision, the value from this provider wins.