Request to add error span tag to Opentelemetry specification

text/0086-support-error-span-tag.md

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+# Addressing discrepancy related to "error" span tag between opentracing and opencensus spec
+
+Adding error span tag from opentracing specification to opentelemtery specification.  
+
+## Motivation
+
+### Why should we make this change? 
+    Tracing backends such as jaeger adhere to opentracing spec [opentracing] (https://github.com/opentracing/specification/blob/master/semantic_conventions.md#span-tags-table). According to opentracing specification [i.e. also how jaeger has implemented] "error" span tag is a boolean value that helps in filtering spans that are in error state. Since opentelemetry does not have "error" span tag in the specification, error span tags are passed as is to the exported backend. According to opentracing spec error tag is boolean, but when zipkin span is translated to jaeger span format "error" tag continues to be a string carrying complete error message. This violates opentracing specification and also breaks jaeger functionality, as jaeger expects "error" to be boolean and "error.message" to be String type that carries description of the error message.    
+####    For example  : 
+        Zipkin spans have "error" tag as a string datatype and they carry short error message. Upon translating this zipkin span to jaeger/opentracing span, ideally we should have 2 tags as shown below
+|SpanTag | DataType | Description|
+------------ | -------------| -------------
+|error|bool|true if and only if the application considers the operation represented by the Span to have failed
+|error.message|string|short description of the error        
+
+####    Zipkin Span : Span with error tag as a string datatype
+|sl/no|SpanFormat | Tag|DataType | Description|SampleSpan|
+------------ | -------------| -------------| -------------| -------------| -------------
+|1|Zipkin|error|string|zipkin span with error tag as a stingType. Refer sample span|{"traceId": "631ef3c9f9250805","id": "631ef3c9f9250805","kind": "SERVER","name": "http:/one","timestamp": 1580830219488000,"duration": 29312,"localEndpoint": {"serviceName": "foo-service", "ipv4": "192.168.1.8", "port": 9001 },"tags": {"error": "Request processing failed; nested exception is org.springframework.web.client.HttpServerErrorException: 500","http.host": "localhost", "http.method": "GET", "http.path": "/one", "http.status_code": "200", "http.url": "http://localhost:9001/one", "mvc.controller.class": "FooController", "mvc.controller.method": "one","spring.instance_id": "192.168.1.8:order-service-sleuth:9001"}}
+|2|Otel Translated Jaeger Span|error|string|above zipkin span translated to jaeger span without changing error tag datatype|    {"traceIdLow":8380789575664951000,"traceIdHigh":0,"spanId":8380789575664951000,"parentSpanId":0,"operationName":"http:/one","flags":0,"startTime":1580830714594000,"duration":79701,"tags":[{"key":"http.url","vType":"STRING","vStr":"http://localhost:9001/one"},{"key":"mvc.controller.class","vType":"STRING","vStr":"FooController"},{"key":"mvc.controller.method","vType":"STRING","vStr":"one"},{"key":"error","vType":"STRING","vStr":"Request processing failed; nested exception is org.springframework.web.client.HttpServerErrorException: 500"},{"key":"http.method","vType":"STRING","vStr":"GET"},{"key":"http.host","vType":"STRING","vStr":"localhost"},{"key":"http.path","vType":"STRING","vStr":"/one"},{"key":"spring.instance_id","vType":"STRING","vStr":"192.168.1.8:order-service-sleuth:9001"},{"key":"http.status_code","vType":"STRING","vStr":"200"},{"key":"span.kind","vType":"STRING","vStr":"server"},{"key":"status.code","vType":"LONG","vLong":2}]}|
+|3|Expected Jaeger Span Translation|error, error.message|bool, string|zipkin error tag should result in 2 span tags. error tag as bool type and error.message as string type|   {"traceIdLow":8380789575664951000,"traceIdHigh":0,"spanId":8380789575664951000,"parentSpanId":0,"operationName":"http:/one","flags":0,"startTime":1580830714594000,"duration":79701,"tags":[{"key":"http.url","vType":"STRING","vStr":"http://localhost:9001/one"},{"key":"mvc.controller.class","vType":"STRING","vStr":"FooController"},{"key":"mvc.controller.method","vType":"STRING","vStr":"one"},{"key":"error","vType":"BOOL","vBool":true},{"key":"error.message","vType":"STRING","vStr":"Request processing failed; nested exception is org.springframework.web.client.HttpServerErrorException: 500"},{"key":"http.method","vType":"STRING","vStr":"GET"},{"key":"http.host","vType":"STRING","vStr":"localhost"},{"key":"http.path","vType":"STRING","vStr":"/one"},{"key":"spring.instance_id","vType":"STRING","vStr":"192.168.1.8:order-service-sleuth:9001"},{"key":"http.status_code","vType":"STRING","vStr":"200"},{"key":"span.kind","vType":"STRING","vStr":"server"},{"key":"status.code","vType":"LONG","vLong":2}]}
+
+##### ISSUE: Refer sl/no 2 above -  JaegerUI stops color coding and filtering such spans, as error is not a bool value.
+
+###### What new value would it bring? 
+Adding below tags to opentelemtery spec would help in non lossy translation of spans
+
+|SpanTag | DataType | Description|
+------------ | -------------| -------------
+|error|bool|true if and only if the application considers the operation represented by the Span to have failed
+|error.message|string|short description of the error 
+
+###### What use cases does it enable?
+This is in accordance with opentracing spec and also fix issues w.r.t lossy translations as explained above.
+
+## Explanation
+This would need a minor change in translation logic across all the formats.
+
+|SpanTag | DataType | Description|
+------------ | -------------| -------------
+|error|bool|true if and only if the application considers the operation represented by the Span to have failed
+|error.message|string|short description of the error 
+
+Refer above motivation section for examples and more details.
+
+## Internal details
+From a technical perspective, how do you propose accomplishing the proposal? In particular, please explain:
+###### How the change would impact and interact with existing functionality
+  This would need a minor change in translation logic across all the formats.