feat: include backtrace first line for better debug info

[xuan-cao-swi]

Propose a solution/improvement for #1669.

Since we are passing the exception class into handle_error, I think it's beneficial to include the last line of the error in the log message for developers to track down the cause.

I believe a full backtrace is too much information to show; however, the end user can always customize their @error_handler to include it.

I didn't add error handling in the encoding function like as_otlp_span because:

1. After rescuing, we need to provide a replacement value for spans: sds.map { |sd| as_otlp_span(sd) }; otherwise, the encode function will raise another type of error, which I think is redundant information.
2. If we provide an empty Opentelemetry::Proto::Trace::V1::Span.new like as_otlp_key_value does, it will raise a single error, but the encode will not throw an error and will export incorrect data (e.g., an empty span), which I think is misleading.

[kaylareopelle]

I like this! Thank you!

As an example, here is the output when I reproduce a known error related to the examples/metrics_sdk/metrics_collect_otlp.rb file when no attributes are given to histogram.record:

E, [2024-08-09T15:03:07.514396 #44506] ERROR -- : OpenTelemetry error: unexpected error in OTLP::MetricsExporter#encode - undefined method `map' for nil - /Users/kreopelle/dev/opentelemetry-ruby/exporter/otlp-metrics/lib/opentelemetry/exporter/otlp/metrics_exporter.rb:274:in `histogram_data_point'

I missed the test failures. Dismissing the review until they're fixed.

[xuan-cao-swi]

Thanks,

If we provide an empty Opentelemetry::Proto::Trace::V1::Span.new like as_otlp_key_value does, it will raise a single error, but the encode will not throw an error and will export incorrect data (e.g., an empty span), which I think is misleading.

I took second thought, and I think it's ok to pass empty Opentelemetry::Proto::Trace::V1::Span or nil, and add check like L126

My second thought will actually make things more complicated. Either way won't break the program because it will be rescued in encode eventually.