Reference A2A.Extension that timestamps requests and responses.

Stamps Message.metadata on the way in and Task.metadata on the way out under the extension's URI:

"https://a2a-protocol.org/extensions/timestamp/v1" => %{
  received_at: 1_700_000_000_000,
  completed_at: 1_700_000_000_042
}

Times are wall-clock milliseconds via System.system_time(:millisecond).

Usage

Configure on the server and (optionally) on the client. Both must declare the URI for the activation to round-trip.

# Server
Bandit.start_link(
  plug: {A2A.Plug,
    agent: MyAgent,
    base_url: "http://localhost:4000",
    extensions: [A2A.Extension.Timestamp]}
)

# Client
client = A2A.Client.new("http://localhost:4000",
  extensions: [A2A.Extension.Timestamp])

{:ok, task} = A2A.Client.send_message(client, "hi")
task.metadata["https://a2a-protocol.org/extensions/timestamp/v1"]
#=> %{"received_at" => 1700000000000, "completed_at" => 1700000000042}

Map keys are atoms on the server (before encoding) and strings on the client (after JSON decode).

What this exercises

This module implements every optional callback of A2A.Extension:

• A2A.Extension.declaration/1 — non-required profile extension.
• A2A.Extension.activate/3 — captures the per-request start time.
• A2A.Extension.handle_request/3 — stamps the inbound message metadata.
• A2A.Extension.handle_response/3 — stamps the outbound task metadata.

Copy it as a starting template for your own profile-style extension.