Requires merge of #3526 to bump core to 0.25.0.

Draft release notes:

google.cloud-logging-1.1.0

• Update google-cloud-core dependency to ~= 0.25.
• Send trace context with logs from web applications (PR Send trace context with logs from web applications #3448, issue Send trace context with logs from web applications #3359)
• Write logs via Stackdriver logging API, rather than writing to /var/log/app_engine/ (PR Stop writing to '/var/log/app_engine/' and write logs to Stackdriver logging API #3410, issue Stop writing to /var/log/app_engine/ #2997)
• Implement flush for cloud logging handlers (PR Implement flush for cloud logging handlers #3413)
• Call start when creating a background thread transport (PR Call start when creating a background thread transport #3412)
• Add monitored resource support to Logging (PR Add MonitoredResource to interface #3386, issues Python Logging API always defaults to Custom logs #2673, Implement monitored resource for logging client #3377)

Not included in notes:

• Re-enable pylint in info-only mode for all packages (Re-enable pylint in info-only mode for all packages #3519)
• Vision semi-GAPIC (Vision semi-GAPIC #3373) (docstring tweaks)
• Remove uncovered / pointless branch. (Remove uncovered / pointless branch. #3445)
• Fixing broken logging system test. (Fixing broken logging system test. #3325)
• Ignore tests (rather than unit_tests) in setup.py files. (Ignore tests (rather than unit_tests) in setup.py files. #3319)
• Overhaul logging background thread transport (Overhaul logging background thread transport #3407)
• Adding check that all setup.py README's are valid RST. (Adding check that **all** setup.py README's are valid RST. #3318)