Bound client-controlled tool/prompt name cardinality on per-request metrics

mcp.server.operation.duration sets gen_ai.tool.name and gen_ai.prompt.name from parsedMCP.ResourceID, which is params.name verbatim from the client's JSON-RPC body (pkg/telemetry/middleware.go, recordOperationDuration). Neither is validated against the server's resolved tool or prompt set, so a client calling arbitrary names grows this metric's series count without bound.

The same pattern exists in vMCP's optimizer: stacklok.vmcp.optimizer.call_tool.{requests,duration} carry tool_name from input.ToolName (pkg/vmcp/server/sessionmanager/factory.go). The not_found outcome is direct evidence that unresolvable names reach that path and are still recorded.

mcp.method.name and http.request.method were bounded to the semconv _OTHER sentinel in #5956, so this is the remaining unbounded surface on per-request metrics. It matters more after #5956 because mcp.server.operation.duration becomes the sole per-method metric once the legacy twins are retired — there is no unaffected alternative to fall back on.

RFC §3.3 cardinality policy: never use an unbounded value as a label on a per-request metric.

Why it wasn't fixed in #5956

Bounding these requires the resolved tool/prompt set, which the telemetry middleware has no access to — it sees a parsed request, not the backend's capability list. Fixing it means either threading capability state into telemetry or resolving the name before the metric is recorded. That's a design change, not a one-line guard.

Interim mitigation (documented)

Drop the attributes with a Prometheus metric_relabel_config if untrusted clients can reach the proxy. See docs/observability.md → Cardinality warning.

Acceptance

• No per-request metric carries a client-controlled tool/prompt name unbounded.
• Unresolvable names record a bounded sentinel, with the raw value available on spans.

Read pkg/telemetry/middleware.go, especially recordOperationDuration, and trace how parsedMCP.ResourceID reaches metric attributes. Then inspect pkg/vmcp/server/sessionmanager/factory.go for the optimizer's tool_name path and review the documented cardinality policy. Done means every per-request metric uses a bounded value for unresolvable tool or prompt names while raw values remain available on spans.