Replace proto with span serialization

[mladjan-gadzic]

• reflection invocation of internal methods removed
  • ILogger used to create LogRecord instead of internal constructor
  • proto serialization is removed and json is used instead per spec https://github.com/open-telemetry/opentelemetry-collector/blob/main/pdata/ptrace/json.go
• example is simplified
• unit tests are added
• manual tests against partial collector are successful

[swythan]

Looks good, but would be good to fix the compile warnings

[swythan]

LGTM