Add metrics showing tracing header outcomes

[schmidtw]

To help monitor if tracing information is propagated it would be helpful to add a metric showing the outcome with regard to the input.

Add a label: trace_info

• none when no trace information is propagated
• both_headers_present when both tracestate & traceparent are present and passed along
• only_traceparent when only a traceparent was present
• invalid when only a tracestate was present (which is invalid according to w3c)