Tracing context and access logging integration

In this PR, we propose the ability to integrate metadata associated with HTTP tracing (e.g. Trace ID) with access logging. For example, suppose a request generates a trace context like the following

{
  "trace_id": "xxx",
  "spans": [.
    {
      "span_id": "yyyy",
      ...
    }
  ]
  ...
}

By providing such a context to a dedicated access logging filter (this filter must be provided optimized for the tracing provider), the following log message is generated, for example.

{"trace_id": "xxx", "message":"..."}

The specific context of this function is explained in detail at here.

cc. @wu-sheng

[optional Relevant Links:]

https://github.com/SkyAPM/cpp2sky/issues/75