PoC for bespoke tracing

[huitseeker]

This keeps Miden's tracing policy, but uses more of the Rust tracing ecosystem for the common parts: tracing::instrument, tracing_error::ErrorLayer, tracing_panic::panic_hook, tracing_subscriber::reload, fmt::Layer, and strum.

Returned errors and panics are now span-level signals with useful context. Returned errors record the full error report on the span and avoid the duplicate exception event from native #[instrument(err)]. Panics become span attributes plus span error status.

Local stdout stays separate from OTel export. User-facing output is a tracing layer with explicit user span/event selection. Selected OTel-side user fields are bridged into miden.user.fields so fmt::Layer can print them.

Runtime filter reload now uses tracing_subscriber::reload. Target policy lives in miden-node-tracing-targets, so the macro and runtime filters share one allowlist. Control-plane events still bypass user filters, but third-party targets stay blocked.

Diff shape:

• Non-test code, config, and docs: +1,320/-888, net +432
• Tests: +760/-60, net +700
• Total: +2,080/-948, net +1,132

New capabilities and their direct tests:

Capability	Tests
Returned errors record one span-level full error report, without the duplicate exception event from native #[instrument(err)]	span::tests::native_instrument_err_uses_top_level_error_and_emits_an_event, span::tests::instrument_macro_records_returned_errors, span::tests::instrument_macro_records_alias_returned_errors_with_err_marker
Panics record span attributes and span error status through tracing-panic control-plane events	internal::tests::control_plane_layer_records_panic_fields_on_current_span, internal::tests::control_plane_layer_bridges_panic_status_to_tracing_error_field
Panics attach to the nearest exported parent when the current child span is disabled or filtered out	internal::tests::control_plane_layer_selects_panic_span_from_event_scope, internal::tests::control_plane_layer_records_panic_on_active_parent_when_child_span_is_disabled, internal::tests::control_plane_layer_skips_scope_child_filtered_from_exporters
Raw panic/control-plane plumbing is hidden from normal output and OTel export	internal::tests::control_plane_event_filter_rejects_tracing_panic_events, exporter::tests::plumbing_layers_do_not_enable_miden_callsites_when_exporters_are_off
Selected OTel-side user fields render in tracing-side formatted output	exporter::stdout::tests::fmt_layer_does_not_render_open_telemetry_span_attributes, exporter::stdout::tests::user_facing_span_data_can_be_rendered_by_fmt_layer
User-facing stdout has explicit span/event selection separate from OTel export	exporter::stdout::tests::layer_logs_user_spans_and_events, exporter::stdout::tests::layer_ignores_non_user_spans_and_events, exporter::stdout::tests::layer_does_not_inherit_user_event_marker_from_span
Runtime filter updates use tracing_subscriber::reload and keep filter readback	filter::tests::dynamic_filter_tracks_current_value, filter::tests::dynamic_filter_set_enables_previously_disabled_callsites, filter::tests::tracing_subscriber_reload_enables_previously_disabled_callsites, exporter::tests::tracing_handle_exposes_filter_accessors
Shared target policy rejects third-party telemetry while allowing control-plane bypasses	target::tests::formats_allowed_targets_as_list, target::tests::parses_allowed_target_path, target::tests::rejects_unknown_target_path, filter::tests::dynamic_filter_rejects_third_party_targets, filter::tests::control_plane_target_bypasses_dynamic_filter

Additional coverage:

Requirement	Tests
Successful instrumented calls leave span status unset	span::tests::instrument_macro_leaves_success_status_unset
OTel fields and objects can be injected into error tracing	span::tests::span_records_fields_with_default_and_override_keys, span::tests::span_records_objects_with_default_and_override_prefixes, span::tests::span_records_user_fields_with_prefixed_keys
Active span selection ignores third-party current spans and handles re-entered spans	internal::tests::active_span_selection_ignores_third_party_current_span, internal::tests::active_span_layer_keeps_outer_enter_active_when_same_span_is_reentered
Async and explicit span tracking keep active-span state correct	internal::tests::active_span_future_tracks_span_only_during_poll, internal::tests::active_span_layer_tracks_public_entered_span, internal::tests::explicit_active_span_guard_does_not_pop_layer_tracked_span
Span filters still apply under the Miden target gate and control-plane wrapper	filter::tests::application_target_gate_still_applies_inside_span_filters, filter::tests::control_plane_filter_wrapper_preserves_span_filters
Target taxonomy rejects component targets and string targets	target::tests::rejects_component_target, target::tests::rejects_target_string