structured logging: revise resource type and info retrieval

[minherz]

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

Describe the solution you'd like
Resource type and additional resource metadata should be populated similarly when logged using Logging agent.
The populated metadata labels should be per resource type and adhere to official documentation

[minherz]

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