feat(telemetry): add OTel standard logs as third signal

[jeanscherf]

Summary

• Wires up a LoggerProvider alongside existing metrics and traces using the shared resource attributes (service.name, region, subaccount, etc.)
• auto_instrument() now sets up all three OTel signals — no app boilerplate changes needed
• Uses LoggingHandler from opentelemetry-instrumentation-logging (not the deprecated SDK one) installed on the root stdlib logger

Changes

• _provider.py: setup_log_provider() + _create_log_exporter() — mirrors _setup_meter_provider() pattern, same grpc/http protocol switch
• auto_instrument.py: one setup_log_provider() call wired in after trace setup
• user-guide.md: new Logging section covering usage, extra={} structured fields, level filtering, trace correlation

Tests

• Mock unit tests for _create_log_exporter, setup_log_provider, and auto_instrument wiring
• End-to-end tests with a real in-memory exporter verifying body, severity, resource attributes, extra={} fields, and trace/span correlation — all run in CI without any external dependencies

App usage

No changes needed. After auto_instrument(), all existing logging.getLogger(...) calls ship logs to the OTel backend automatically:

auto_instrument()  # sets up traces, metrics, and logs

logger = logging.getLogger(__name__)
logger.info("Request handled", extra={"tenant_id": tid})