Harden Pylon stats collection for Dynamo, NIM, and vLLM workloads

Context

Pylon’s stats collection is functional today, but it should be hardened before production or delivered as a fast-follow. The Dynamo stats contract does not exist yet, and Pylon also needs to work robustly with NIM and vLLM workloads.

Goals

• Define and document the Dynamo stats contract and its compatibility expectations.
• Harden Pylon’s stats collection, parsing, validation, and error handling.
• Ensure stats collection behaves reliably across NIM and vLLM.
• Prevent missing or malformed stats from affecting the primary workload path.
• Add focused tests and observability for collection failures and contract mismatches.

Delivery

Assess whether this should be completed pre-production or tracked as a fast-follow based on the contract and compatibility work required.

Requested based on feedback from Barry Greengus.