add support for OTLP logging/tracing (#599)

- adds `zentelemetry` project which houses new functionality for serializing logs and traces in OpenTelemetry Protocol format (OTLP)
- moved existing stats functionality from `zencore` to `zentelemetry`
- adds `TRefCounted<T>` for vtable-less refcounting
- adds `MemoryArena` class which allows for linear allocation of memory from chunks
- adds `protozero` which is used to encode OTLP protobuf messages

1 files changed, 166 insertions, 0 deletions

diff --git a/src/zentelemetry/otelprotozero.h b/src/zentelemetry/otelprotozero.h
new file mode 100644
index 000000000..8ad69e766
--- /dev/null
+++ b/src/zentelemetry/otelprotozero.h
@@ -0,0 +1,166 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#pragma once
+
+#include <protozero/types.hpp>
+
+//////////////////////////////////////////////////////////////////////////
+//
+//  OTEL .proto definitions, for reference
+//
+
+#if 0
+
+// clang-format off
+
+////////////////////////////////////////////////////////////////////////
+// resource/v1/resource.proto
+//
+
+// Resource information.
+message Resource {
+  // Set of attributes that describe the resource.
+  // Attribute keys MUST be unique (it is not allowed to have more than one
+  // attribute with the same key).
+  //
+  // The attribute values SHOULD NOT contain empty values.
+  // The attribute values SHOULD NOT contain bytes values.
+  // The attribute values SHOULD NOT contain array values different than array of string values, bool values, int values,
+  // double values.
+  // The attribute values SHOULD NOT contain kvlist values.
+  // The behavior of software that receives attributes containing such values can be unpredictable.
+  // These restrictions can change in a minor release.
+  // The restrictions take origin from the OpenTelemetry specification:
+  // https://github.com/open-telemetry/opentelemetry-specification/blob/v1.47.0/specification/common/README.md#attribute.
+  repeated opentelemetry.proto.common.v1.KeyValue attributes = 1;
+
+  // The number of dropped attributes. If the value is 0, then
+  // no attributes were dropped.
+  uint32 dropped_attributes_count = 2;
+
+  // Set of entities that participate in this Resource.
+  //
+  // Note: keys in the references MUST exist in attributes of this message.
+  //
+  // Status: [Development]
+  repeated opentelemetry.proto.common.v1.EntityRef entity_refs = 3;
+}
+
+/////////////////////////////////////////////////////////////////////////
+// common/v1/commmon.proto
+//
+
+// Represents any type of attribute value. AnyValue may contain a
+// primitive value such as a string or integer or it may contain an arbitrary nested
+// object containing arrays, key-value lists and primitives.
+message AnyValue {
+  // The value is one of the listed fields. It is valid for all values to be unspecified
+  // in which case this AnyValue is considered to be "empty".
+  oneof value {
+    string string_value = 1;
+    bool bool_value = 2;
+    int64 int_value = 3;
+    double double_value = 4;
+    ArrayValue array_value = 5;
+    KeyValueList kvlist_value = 6;
+    bytes bytes_value = 7;
+  }
+}
+
+// ArrayValue is a list of AnyValue messages. We need ArrayValue as a message
+// since oneof in AnyValue does not allow repeated fields.
+message ArrayValue {
+  // Array of values. The array may be empty (contain 0 elements).
+  repeated AnyValue values = 1;
+}
+
+// KeyValueList is a list of KeyValue messages. We need KeyValueList as a message
+// since `oneof` in AnyValue does not allow repeated fields. Everywhere else where we need
+// a list of KeyValue messages (e.g. in Span) we use `repeated KeyValue` directly to
+// avoid unnecessary extra wrapping (which slows down the protocol). The 2 approaches
+// are semantically equivalent.
+message KeyValueList {
+  // A collection of key/value pairs of key-value pairs. The list may be empty (may
+  // contain 0 elements).
+  // The keys MUST be unique (it is not allowed to have more than one
+  // value with the same key).
+  repeated KeyValue values = 1;
+}
+
+// Represents a key-value pair that is used to store Span attributes, Link
+// attributes, etc.
+message KeyValue {
+  // The key name of the pair.
+  string key = 1;
+
+  // The value of the pair.
+  AnyValue value = 2;
+}
+
+// InstrumentationScope is a message representing the instrumentation scope information
+// such as the fully qualified name and version. 
+message InstrumentationScope {
+  // A name denoting the Instrumentation scope.
+  // An empty instrumentation scope name means the name is unknown.
+  string name = 1;
+
+  // Defines the version of the instrumentation scope.
+  // An empty instrumentation scope version means the version is unknown.
+  string version = 2;
+
+  // Additional attributes that describe the scope. [Optional].
+  // Attribute keys MUST be unique (it is not allowed to have more than one
+  // attribute with the same key).
+  repeated KeyValue attributes = 3;
+
+  // The number of attributes that were discarded. Attributes
+  // can be discarded because their keys are too long or because there are too many
+  // attributes. If this value is 0, then no attributes were dropped.
+  uint32 dropped_attributes_count = 4;
+}
+
+// clang-format on
+#endif
+
+//////////////////////////////////////////////////////////////////////////
+
+namespace zen::otel {
+
+enum class KeyValueList : protozero::pbf_tag_type
+{
+	repeated_KeyValue_values = 1
+};
+
+enum class KeyValue : protozero::pbf_tag_type
+{
+	string_key	   = 1,
+	AnyValue_value = 2
+};
+
+enum class Resource : protozero::pbf_tag_type
+{
+	repeated_KeyValue_attributes	= 1,
+	uint32_dropped_attributes_count = 2,
+	repeated_EntityRef_entity_refs	= 3
+};
+
+enum class AnyValue : protozero::pbf_tag_type
+{
+	string_string_value		  = 1,
+	bool_bool_value			  = 2,
+	int64_int_value			  = 3,
+	double_double_value		  = 4,
+	ArrayValue_array_value	  = 5,
+	KeyValueList_kvlist_value = 6,
+	bytes_bytes_value		  = 7
+};
+
+enum class InstrumentationScope : protozero::pbf_tag_type
+{
+	string_name						= 1,
+	string_version					= 2,
+	repeated_KeyValue_attributes	= 3,
+	uint32_dropped_attributes_count = 4
+};
+
+}  // namespace zen::otel