out_stackdriver: convert invalid utf-8 character to replacement string

[shuaich]

Is your feature request related to a problem? Please describe.

Google Cloud Logging API requires strings to be UTF-8 encoded[1]. Log entries with invalid UTF-8 characters fail to be ingested when partial_success is enabled, otherwise the whole WriteLogEntries request is dropped.

The out_google_cloud plugin in fluentd provides a feature that converts invalid UTF-8 characters to replacement strings[2].

I propose to add the same feature available in out_stackdriver plugin.

[1]https://cloud.google.com/logging/docs/reference/v2/rpc/google.logging.v2#google.logging.v2.LoggingServiceV2.WriteLogEntries
[2]https://github.com/GoogleCloudPlatform/fluent-plugin-google-cloud/blob/master/lib/fluent/plugin/out_google_cloud.rb#L1873C9-L1873C24

Describe the solution you'd like

To add two configuration parameters: coerce_to_utf8 and non_utf8_replacement_string. The expected behaviour is:

1. coerce_to_utf8 defaults to false, non_utf8_replacement_string defaults to a space character ' '.
2. When coerce_to_utf8 is enabled, convert invalid UTF-8 characters to configured replacement string for all string fields in LogEntry
3. When coerce_to_utf8 is disabled, skip the conversion

Describe alternatives you've considered

Additional context

[shuaich]

Fluentbit strips away remaining string containing the invalid UTF-8 chracters, relevant issues are #2142 #5035