[receiver/splunkhec] Fix the double encoding of JSON responses (open-telemetry#27606)

**Description:**
We would double-encode JSON objects as strings. We typically have
encoded strings as JSON strings as static responses to issues
encountered during data processing. Along the way, we adopted JSON
objects to match the Splunk Enterprise API. However, we didn't stop
encoding as JSON strings, resulting in double encoded JSON strings.

**Link to tracking Issue:**
Fixes open-telemetry#27604 

**Testing:**
Unit test changes.

Author: atoulme

.chloggen/json_error_codes.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: splunkhecreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Do not encode JSON response objects as string.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27604]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/splunkhecreceiver/receiver.go

@@ -34,16 +34,16 @@ const (
 
 	responseOK                        = `{"text": "Success", "code": 0}`
 	responseHecHealthy                = `{"text": "HEC is healthy", "code": 17}`
-	responseInvalidMethod             = `Only "POST" method is supported`
-	responseInvalidEncoding           = `"Content-Encoding" must be "gzip" or empty`
+	responseInvalidMethod             = `"Only \"POST\" method is supported"`
+	responseInvalidEncoding           = `"\"Content-Encoding\" must be \"gzip\" or empty"`
 	responseInvalidDataFormat         = `{"text":"Invalid data format","code":6}`
 	responseErrEventRequired          = `{"text":"Event field is required","code":12}`
 	responseErrEventBlank             = `{"text":"Event field cannot be blank","code":13}`
-	responseErrGzipReader             = "Error on gzip body"
-	responseErrUnmarshalBody          = "Failed to unmarshal message body"
-	responseErrInternalServerError    = "Internal Server Error"
-	responseErrUnsupportedMetricEvent = "Unsupported metric event"
-	responseErrUnsupportedLogEvent    = "Unsupported log event"
+	responseErrGzipReader             = `"Error on gzip body"`
+	responseErrUnmarshalBody          = `"Failed to unmarshal message body"`
+	responseErrInternalServerError    = `"Internal Server Error"`
+	responseErrUnsupportedMetricEvent = `"Unsupported metric event"`
+	responseErrUnsupportedLogEvent    = `"Unsupported log event"`
 	responseErrHandlingIndexedFields  = `{"text":"Error in handling indexed fields","code":15,"invalid-event-number":%d}`
 	responseNoData                    = `{"text":"No data","code":5}`
 	// Centralizing some HTTP and related string constants.
@@ -58,18 +58,18 @@ var (
 	errInvalidMethod          = errors.New("invalid http method")
 	errInvalidEncoding        = errors.New("invalid encoding")
 
-	okRespBody                = initJSONResponse(responseOK)
-	eventRequiredRespBody     = initJSONResponse(responseErrEventRequired)
-	eventBlankRespBody        = initJSONResponse(responseErrEventBlank)
-	invalidEncodingRespBody   = initJSONResponse(responseInvalidEncoding)
-	invalidFormatRespBody     = initJSONResponse(responseInvalidDataFormat)
-	invalidMethodRespBody     = initJSONResponse(responseInvalidMethod)
-	errGzipReaderRespBody     = initJSONResponse(responseErrGzipReader)
-	errUnmarshalBodyRespBody  = initJSONResponse(responseErrUnmarshalBody)
-	errInternalServerError    = initJSONResponse(responseErrInternalServerError)
-	errUnsupportedMetricEvent = initJSONResponse(responseErrUnsupportedMetricEvent)
-	errUnsupportedLogEvent    = initJSONResponse(responseErrUnsupportedLogEvent)
-	noDataRespBody            = initJSONResponse(responseNoData)
+	okRespBody                = []byte(responseOK)
+	eventRequiredRespBody     = []byte(responseErrEventRequired)
+	eventBlankRespBody        = []byte(responseErrEventBlank)
+	invalidEncodingRespBody   = []byte(responseInvalidEncoding)
+	invalidFormatRespBody     = []byte(responseInvalidDataFormat)
+	invalidMethodRespBody     = []byte(responseInvalidMethod)
+	errGzipReaderRespBody     = []byte(responseErrGzipReader)
+	errUnmarshalBodyRespBody  = []byte(responseErrUnmarshalBody)
+	errInternalServerError    = []byte(responseErrInternalServerError)
+	errUnsupportedMetricEvent = []byte(responseErrUnsupportedMetricEvent)
+	errUnsupportedLogEvent    = []byte(responseErrUnsupportedLogEvent)
+	noDataRespBody            = []byte(responseNoData)
 )
 
 // splunkReceiver implements the receiver.Metrics for Splunk HEC metric protocol.
@@ -464,15 +464,6 @@ func (r *splunkReceiver) handleHealthReq(writer http.ResponseWriter, _ *http.Req
 	_, _ = writer.Write([]byte(responseHecHealthy))
 }
 
-func initJSONResponse(s string) []byte {
-	respBody, err := jsoniter.Marshal(s)
-	if err != nil {
-		// This is to be used in initialization so panic here is fine.
-		panic(err)
-	}
-	return respBody
-}
-
 func isFlatJSONField(field interface{}) bool {
 	switch value := field.(type) {
 	case map[string]interface{}: