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](https://github.com/SkyAPM/cpp2sky/issues/75).

cc. @wu-sheng

[optional *Relevant Links*:]
> https://github.com/SkyAPM/cpp2sky/issues/75