Expose metadata in TraceProcessor

This change addresses issue #399 (https://github.com/google/perfetto/issues/399).

Modifications:
- The `TraceProcessor` class in `python/perfetto/trace_processor/api.py` now stores metadata when a trace URI is resolved.
- A new `get_metadata()` method is added to `TraceProcessor` to allow you to access this metadata.
- I added a new test case, `test_trace_processor_metadata`, to `python/test/api_integrationtest.py` to verify this new functionality. It checks that metadata is correctly populated when using a URI resolver and that it behaves as expected (returns None) when a direct file path is used.