Run dart_code_metrics on CI and fix current warnings

[marandaneto]

https://pub.dev/packages/dart_code_metrics

Similar to flutter/flutter#104550

[marandaneto]

lib/src/client_reports/client_report.dart:
⚠ constructor ClientReport has unnecessary nullable parameters
(this.timestamp)
at /Users/.../Github/sentry-dart/dart/lib/src/client_reports/client_report.dart:8:3

lib/src/enricher/io_enricher_event_processor.dart:
⚠ method _getRuntimes has unnecessary nullable parameters
(List? runtimes)
at /Users/.../Github/sentry-dart/dart/lib/src/enricher/io_enricher_event_processor.dart:54:3

lib/src/http_client/breadcrumb_client.dart:
⚠ constructor BreadcrumbClient has unnecessary nullable parameters
(Client? client, Hub? hub)
at /Users/.../Github/sentry-dart/dart/lib/src/http_client/breadcrumb_client.dart:45:3

lib/src/http_client/failed_request_client.dart:
⚠ constructor FailedRequestClient has unnecessary nullable parameters
(Client? client, Hub? hub)
at /Users/.../Github/sentry-dart/dart/lib/src/http_client/failed_request_client.dart:67:3
⚠ method _captureEvent has unnecessary nullable parameters
(required Object? exception)
at /Users/.../Github/sentry-dart/dart/lib/src/http_client/failed_request_client.dart:149:3

lib/src/http_client/tracing_client.dart:
⚠ constructor TracingClient has unnecessary nullable parameters
(Client? client, Hub? hub)
at /Users/.../Github/sentry-dart/dart/lib/src/http_client/tracing_client.dart:11:3

lib/src/sentry_envelope_header.dart:
⚠ constructor SentryEnvelopeHeader has unnecessary nullable parameters
(this.eventId, this.sdkVersion)
at /Users/.../Github/sentry-dart/dart/lib/src/sentry_envelope_header.dart:7:3

✖ total declarations (functions, methods and constructors) with unnecessary nullable parameters - 7

[marandaneto]

this package is DISCONTINUED