fix(observability): propagate component span into spawned tasks

[gwenaskell]

Summary

Several components spawn background tokio tasks (or run work on detached tasks via concurrent_map) without carrying the current tracing span. Vector's internal metrics/logs derive their component tags (component_id, component_kind, component_type) from that span, so any telemetry emitted from those tasks was missing its component tags.

This PR preserves the owning component's span across the spawn boundary, by wrapping spawned futures with in_current_span() (and, for the Datadog logs/metrics sinks that drive concurrent_map directly, instrumenting the mapped future inside the closure where the sink span is current).

Notes:

• ConcurrentMap, and spawn_named, do not instrument any spawned future themselves — more generally, the responsibility of instrumenting a future should lie with the code that declares the future, not the one that spawns it
• The fanout detached-send task is intentionally left un-instrumented, with a comment: it drains a sink that was just detached from the topology, so it is unrelated to the upstream component that owns the fanout; instrumenting it would mis-tag the work with the wrong component.

Examples of internal telemetry that was previously mis-tagged and is now correct

• datadog_logs sink — component_discarded_events_total (via ComponentEventsDropped, emitted when an event is too large to encode) is produced inside the request-builder future spawned by concurrent_map. Previously it was emitted with no component tags; now it carries the sink's component_id/component_kind/component_type.
• gcp_pubsub source — each pull stream runs on a per-stream task spawned in start_one. Errors emitted there (e.g. StreamClosedError → component_errors_total + component_discarded_events_total, and connection/streaming errors → component_errors_total) were untagged and are now attributed to the source component.
• splunk_hec sinks — the indexer-acknowledgement handling task emits SplunkResponseParseError / SplunkIndexerAcknowledgementUnavailableError (component_errors_total); these were untagged and are now correctly attributed to the sink.

Vector configuration

N/A — no configuration changes.

How did you test this PR?

• cargo check -p vector --lib passes.
• Manual audit of each spawn site to confirm the captured span belongs to the spawned work (and to identify the fanout case that should not be instrumented).

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

[bruceg]

LGTM

[bruceg]

Would it be helpful to have a spawn_in_current_task wrapper for this common operation? It's a relatively trivial amount of code duplication, but it would simplify searching for spawns that are missing spans and potentially even block calling tokio::spawn directly without proper annotations.