import metrics writes app-authored metric samples through the Vary
Server runtime contract. The runtime supplies the metrics endpoint and
the workload identity token at launch; the server derives the emitting
app from the token rather than from app-provided fields.
import metrics
metrics.write("requests_total", 1.0, "count", {"route": "/api/v1/issues"})
metrics.scalar("queue_depth", 12.0, "count")
| Function | Purpose |
|---|---|
write(metric_name, value, unit, tags) | One sample with string tags. Defaults: count, {}. |
scalar(metric_name, value, unit) | Same as write with no tags. |
value is a Float. unit is a string; common values are count,
seconds, bytes, and ratio. tags is a Dict[Str, Str].
All errors subclass MetricError:
| Error | Cause |
|---|---|
MetricBootstrapError | VARY_METRICS_URL or VARY_CONFIG_TOKEN_PATH is unset. |
MetricAuthorizationError | The workload identity token was rejected by the server. |
MetricValidationError | Invalid metric name, unit, tag key, or tag value. |
MetricCardinalityError | The server applied a cardinality cap to the metric. |
Treat metric writes as best-effort observability: catch MetricError at
the call site and continue. Do not fail a request because a metric write
failed.
Via exposes app-authored metrics through the operator surface
(via metrics ...) and through the per-app status block. App-side
metrics share the same retention policy as runtime metrics; the platform
applies cardinality limits before storage to prevent a runaway tag from
overwhelming the host.
Metric names should be lowercase, dotted or underscored, and stable across releases. Reserve high-cardinality dimensions (user ids, request ids) for log fields or audit events instead of metric tags.