Struct MetricsDispenser 

pub struct MetricsDispenser { /* private fields */ }

Wraps an inner OpDispenser to publish per-cycle synthetic metrics declared in the op template’s metrics: map (SRD-40b §6).

Per-cycle responsibilities (in order, matching SRD-40b §5.2 → §6 pipeline):

1. Await the inner dispenser’s execute. With a crate::wrappers::ResultDispenser in the wrapper stack between the inner adapter and this one, declared result: wires are already written through ctx.wires.write to the per-fiber kernel by the time we run.
2. For each declared metric, read the value through ctx.wires.get(name) (bare-binding-name canonical form per SRD-40b §1). The read pulls fresh through the eval cone so any computed output (e.g. row_count := count) reflects this cycle’s value.
3. Apply the optional [metric_format::FormatSpec] sanitiser to round to the configured precision (Phase B).
4. Dispatch to the kind-specific instrument record method (SRD-40b §6.1):
   • [MetricKind::Gauge] → [ValueGauge::set] (f64).
   • [MetricKind::Histogram] → [Histogram::record] (truncated to u64 after format rounding).
   • [MetricKind::Counter] → [Counter::inc_by] (u64); non-positive values warn and skip — counters are monotonic by definition.

Non-bare-name expressions (factor * 2.0, if(...), …) are deferred — the wrap step errors when spec.value is not a bare identifier.

Implementations

impl MetricsDispenser

pub fn wrap( inner: Arc<dyn OpDispenser>, metrics: &HashMap<String, MetricSpec>, component: &mut Component, component_arc: &Arc<RwLock<Component>>, fx: &mut ScopeFixture, ) -> Result<Arc<dyn OpDispenser>, String>

Wrap an inner dispenser with synthetic-metric publication for the op template’s metrics: declarations.

Init steps (SRD-40b §6 init):

1. Empty declaration → return inner unchanged. No overhead for ops that don’t publish synthetic metrics.
2. For each (name, spec), allocate the kind-specific instrument and register it on the component via Component::register_instrument. A duplicate-family collision (§7.2) errors here, before any cycle runs.
3. Pre-parse the optional format: string into a [FormatSpec].

component is borrowed mutably so register_instrument can claim the family slot atomically with the instrument allocation. The same Arc<...> is held both on the component (for cadence-reporter capture) and in the returned dispenser’s slots (for per-cycle record).

Trait Implementations

impl OpDispenser for MetricsDispenser

fn execute<'a>( &'a self, cycle: u64, ctx: &'a ExecCtx<'a>, ) -> Pin<Box<dyn Future<Output = Result<OpResult, ExecutionError>> + Send + 'a>>

Execute an operation for the given cycle.

fn inner_dispenser(&self) -> Option<&dyn OpDispenser>

Returns the wrapped dispenser when this is a wrapper, None when this is a leaf (the adapter’s base dispenser — e.g. CQL raw / prepared / batch, HTTP, stdout, plotter, …).

fn describe(&self) -> Option<String>

One-line description of the op this dispenser represents — typically the statement text / request shape with bind-point placeholders left unresolved ({table}, {key}, ?, …).

fn describe_resolved(&self, wires: &dyn WireSource) -> Option<String>

Render the actual op the dispenser would fire for the given cycle — the dryrun-equivalent view of the statement after every bind point is interpolated.

fn canonical_kernel(&self) -> Option<&Arc<dyn Kernel>>

SRD-68 invariant I-3 — the dispenser’s canonical Polydat Kernel, established at construction by map_op from its parent — the parent itself, or a kernel bound under it. Any engine: the executor finds the program to bind per fiber by program_id. Returns None for dispensers that don’t own a kernel (adapters with no Polydat needs, or wrappers that delegate to an inner dispenser).

fn adapter_metrics(&self) -> Vec<(String, Labels, MetricValue)>

Snapshot adapter-specific metrics for inclusion in the capture snapshot.

fn status_counters(&self) -> Vec<(&str, u64)>

Adapter-specific status line entries (cumulative, non-destructive read).

fn rows_per_op(&self) -> usize

The op’s uniform per-invocation cursor consumption — how many consecutive wire ordinals one execute call reads and covers.

impl WrappingDispenser for MetricsDispenser