structured logging: add trace info discovery from Http request info

[minherz]

Is your feature request related to a problem? Please describe.
If not defined, the trace and spanId fields in LogEntry should be auto-populated when writing log entries to backend

Describe the solution you'd like
When Http request info is available and it has the X-Cloud-Trace-Context header, the tracing info should be retrieved (following the format) and populated.

[minherz]

Related issues #685, #686, #687, #688, #689, #690, #691

[minherz]

The #687 implements capturing the tracing information in the Context class. The tracing info is captured from the Http servlet request based on W3C trace context header or Google Cloud trace context header. Alternatively, the information can be set manually using ContextHandler class.