[metrics]: Allow request metadata as Prometheus labels - #149
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Proxy metrics could only be sliced by dimensions the proxy itself knows: the method, the upstream, the outcome. An operator running one proxy for several tenants or clusters has no way to attribute traffic to them, because the thing that identifies them travels in request metadata the collectors never see. Add a metrics.tags list, written as "<header>:<label>", that carries an inbound metadata value onto the metrics emitted while serving a request. The gateway's own request metrics, the routing decision, and the vault operations all gain the configured labels. Things that aren't request based are is left as is. `crypto.Observer`, for example, which has no request context, and has some logic in a go routine with no request available at all. Having 2 different headers mapping to the same label is a startup (validation) error, since Prometheus would panic at runtime with two values set for the same one. There's also some validation for the header and label names to keep them inline with gRPC/Prometheus requirements and ensure they're caught at startup and not at runtime. gRPC binary metadata (ending in `-bin`) are not allowed. Finally, gRPC metadata is a `map[string][]string` where each value is the ordered set of values set for the key. In the case that multiple values exist, the last value is used as the label.
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Proxy metrics could only be sliced by dimensions the proxy itself knows: the method, the upstream, the outcome. An operator running a single proxy for several tenants or clusters has no way to attribute traffic to them because the identifiers that identify them travel in request metadata that the collectors never see.
Add a
:", that carries an inbound metadata value onto the metrics emitted while serving a request. The gateway's own request metrics, the routing decision, and the vault operations all gain the configured labels.metrics.tagslist, written as "Things that aren't request-based are left as is.
crypto.Observer, for example, has no request context and runs some logic in a goroutine with no request available, so nothing is added there.Having 2 different headers mapping to the same label is a startup (validation) error, since Prometheus would panic at runtime with two values set for the same one. There's also validation for the header and label names to keep them in line with gRPC/Prometheus requirements and ensure issues are caught at startup rather than at runtime. gRPC binary metadata (ending in
-bin) is not allowed.Finally, gRPC metadata is a
map[string][]stringwhere each value is the ordered set of values set for the key. If multiple values exist, the last value is used as the label.