POST /v3/instruments/:id/interest-rates

Append one interest-rate observation. The CLI is the "client"
provider — `provider` is set to "client" automatically.

Path params:
  id (required) — Instrument identifier (Mongo ObjectID).

Request:
{
  "data": {
    "type": "interest-rate",
    "attributes": {
      "date":  "string, ISO 8601 (required)",
      "value": "number (required) — annual rate as a decimal (0.05 = 5%)"
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "interest-rate",
    "attributes": {
      "date":     "ISO 8601 date",
      "value":    "number",
      "provider": "client"
    }
  }
}