Per-RPC Error Logging

[westarle]

Logs should be emitted for errors on HTTP or gRPC requests, attached to each T4 span. In addition to common transport level attributes like response code and local error messages, these logs have the following attributes:

• error.type e.g. RATE_LIMIT_EXCEEDED
• gcp.error.domain e.g. "pubsub.googleapis.com"
• gcp.error.metadata.<key> e.g. gcp.error.metadata.zone: "us-east1-a"`

These values should be parsed from the ErrorInfo attached to the response as error, if any.

I suggest they are logged at "INFO" since we will be recording them on retryable requests.