Network telemetry improvements

[waltjones]

Fills in some gaps in network telemetry capture, and adds scrubbing for request/response JSON bodies and headers.

Enable telemetry capture for:

• Fetch request headers
• Fetch response headers
• Fetch response body
• XHR request headers

Enable scrubbing:

• Detect fetch request/response content type
• Detect XHR request/response content type
• Scrub request/response headers (XHR and fetch)
• Scrub request/response JSON bodies (XHR and fetch)

Some parts of the Fetch API are missing on IE. All features were tested for IE compat and graceful degradation.