Add origin_offload metric


The Real-Time and Historical Stats responses now include a new metric origin_offload. The description of origin_offload was also updated for the Domain Inspector APIs. For Domain Inspector, the metric was previously computed as the ratio of response bytes delivered from the edge compared to what is delivered from origin, between 0 and 1. (edge_resp_body_bytes + edge_resp_header_bytes) / (origin_fetch_resp_body_bytes + origin_fetch_resp_header_bytes + edge_resp_body_bytes + edge_resp_header_bytes).

Prior change: Go SDK 1.3.2

Following change: Correct the use_tls field to be a string type