[EXPORTER] Elastic Search exporter follow ECS guidelines (open-telemetry#3107)

* [EXPORTERS]: elastic search log message within `message` key instead of
`body`

According to ECS logging reference
https://www.elastic.co/guide/en/ecs/8.11/ecs-base.html#field-message

Refs open-telemetry#3091

* [EXPORTERS]: elastic search set severity within `log.level` key instead of
    `severity`

According to ECS logging reference
https://www.elastic.co/guide/en/ecs/8.11/ecs-log.html#field-log-level

Refs open-telemetry#3091

* [EXPORTERS]: elastic search set timestamp within `@timestamp` instead of
`timestamp`

Also changes the format to be a Date string.

According to ECS logging reference
https://www.elastic.co/guide/en/ecs/8.11/ecs-base.html#field-timestamp

Refs open-telemetry#3091

* [EXPORTERS]: elastic search set instrumentation scope within `log.logger` instead of
`name`

According to ECS logging reference
https://www.elastic.co/guide/en/ecs/8.11/ecs-log.html#field-log-logger

Refs open-telemetry#3091

* EXPORTERS]: elastic search recorable constructor sets `ecs.version` field
to 8.11.0

According to ECS guidelines this field is mandatory
https://www.elastic.co/guide/en/ecs/8.11/ecs-guidelines.html

Refs open-telemetry#3091

* [EXPORTERS]: elastic search put attributes in json root instead of under
`attributes`

This allows user to set other fields that are part of the [ECS log documentation](https://www.elastic.co/guide/en/ecs/8.11/ecs-log.html).
For instance, it allows to have an attribute with key `log.file`, that
will, thanks to `nlohmann::json`, appear as :
```
{
  "log": {
    "file": "xxx"
  }
}
```

Closes open-telemetry#3091

Author: ShadowMaxLeb

exporters/elasticsearch/include/opentelemetry/exporters/elasticsearch/es_log_recordable.h

@@ -42,6 +42,8 @@ class ElasticSearchRecordable final : public sdk::logs::Recordable
   void WriteValue(const opentelemetry::common::AttributeValue &value, const std::string &name);
 
 public:
+  ElasticSearchRecordable() noexcept;
+
   /**
    * Returns a JSON object contain the log information
    */

exporters/elasticsearch/src/es_log_recordable.cc

@@ -197,6 +197,11 @@ void ElasticSearchRecordable::WriteValue(const opentelemetry::common::AttributeV
   }
 }
 
+ElasticSearchRecordable::ElasticSearchRecordable() noexcept : sdk::logs::Recordable()
+{
+  json_["ecs"]["version"] = "8.11.0";
+}
+
 nlohmann::json ElasticSearchRecordable::GetJSON() noexcept
 {
   return json_;
@@ -205,7 +210,38 @@ nlohmann::json ElasticSearchRecordable::GetJSON() noexcept
 void ElasticSearchRecordable::SetTimestamp(
     opentelemetry::common::SystemTimestamp timestamp) noexcept
 {
-  json_["timestamp"] = timestamp.time_since_epoch().count();
+  const std::chrono::system_clock::time_point timePoint{timestamp};
+
+  // If built with with at least cpp 20 then use std::format
+  // Otherwise use the old style to format the timestamp in UTC
+#if __cplusplus >= 202002L
+  const std::string dateStr = std::format("{:%FT%T%Ez}", timePoint);
+#else
+  const static int dateToSecondsSize = 19;
+  const static int millisecondsSize  = 8;
+  const static int timeZoneSize      = 1;
+  const static int dateSize          = dateToSecondsSize + millisecondsSize + timeZoneSize;
+
+  std::time_t time = std::chrono::system_clock::to_time_t(timePoint);
+  std::tm tm       = *std::gmtime(&time);
+
+  char bufferDate[dateSize];  // example: 2024-10-18T07:26:00.123456Z
+  std::strftime(bufferDate, sizeof(bufferDate), "%Y-%m-%dT%H:%M:%S", &tm);
+  auto microseconds =
+      std::chrono::duration_cast<std::chrono::microseconds>(timePoint.time_since_epoch()) %
+      std::chrono::seconds(1);
+
+  char bufferMilliseconds[millisecondsSize];
+  std::snprintf(bufferMilliseconds, sizeof(bufferMilliseconds), ".%06ld",
+                static_cast<long>(microseconds.count()));
+
+  std::strcat(bufferDate, bufferMilliseconds);
+  std::strcat(bufferDate, "Z");
+
+  const std::string dateStr(bufferDate);
+#endif
+
+  json_["@timestamp"] = dateStr;
 }
 
 void ElasticSearchRecordable::SetObservedTimestamp(
@@ -216,23 +252,25 @@ void ElasticSearchRecordable::SetObservedTimestamp(
 
 void ElasticSearchRecordable::SetSeverity(opentelemetry::logs::Severity severity) noexcept
 {
+  auto &severityField = json_["log"]["level"];
+
   // Convert the severity enum to a string
   std::uint32_t severity_index = static_cast<std::uint32_t>(severity);
   if (severity_index >= std::extent<decltype(opentelemetry::logs::SeverityNumToText)>::value)
   {
     std::stringstream sout;
     sout << "Invalid severity(" << severity_index << ")";
-    json_["severity"] = sout.str();
+    severityField = sout.str();
   }
   else
   {
-    json_["severity"] = opentelemetry::logs::SeverityNumToText[severity_index];
+    severityField = opentelemetry::logs::SeverityNumToText[severity_index];
   }
 }
 
 void ElasticSearchRecordable::SetBody(const opentelemetry::common::AttributeValue &message) noexcept
 {
-  WriteValue(message, "body");
+  WriteValue(message, "message");
 }
 
 void ElasticSearchRecordable::SetTraceId(const opentelemetry::trace::TraceId &trace_id) noexcept
@@ -275,7 +313,7 @@ void ElasticSearchRecordable::SetAttribute(
     nostd::string_view key,
     const opentelemetry::common::AttributeValue &value) noexcept
 {
-  WriteKeyValue(key, value, "attributes");
+  WriteValue(value, key.data());
 }
 
 void ElasticSearchRecordable::SetResource(
@@ -291,7 +329,7 @@ void ElasticSearchRecordable::SetInstrumentationScope(
     const opentelemetry::sdk::instrumentationscope::InstrumentationScope
         &instrumentation_scope) noexcept
 {
-  json_["name"] = instrumentation_scope.GetName();
+  json_["log"]["logger"] = instrumentation_scope.GetName();
 }
 
 }  // namespace logs