Switch to absl options/variant to support earlier C++.

exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_exporter_options.h

@@ -3,12 +3,11 @@
 
 #pragma once
 
+#include "absl/types/optional.h"
+#include "absl/types/variant.h"
 #include "opentelemetry/exporters/otlp/otlp_environment.h"
 
 #include <memory>
-// TODO: These requires C++17, is that OK? It seems like we need to support C++0x?
-#include <optional>
-#include <variant>
 
 OPENTELEMETRY_BEGIN_NAMESPACE
 namespace exporter
@@ -17,7 +16,7 @@ namespace otlp
 {
 
 // This type allows the variants to be default-constructed to neither type.
-// This avoids needing deeper template if std::optional<std::variant<>>.
+// This avoids needing deeper template if absl::optional<absl::variant<>>.
 struct Unset {};
 
 // TODO: Check if this strong-typing is ok. It seems nice, and allows variant typing.
@@ -35,13 +34,13 @@ struct FileContents {
 struct SSLCredentials {
   // Path to or contents of a .pem file of the root certs used to verify the service side
   // certificate.
-  std::variant<Unset, FilePath, FileContents> pem_root_certs;
+  absl::variant<Unset, FilePath, FileContents> pem_root_certs;
 
   // Path to or contents of a .pem file containing the client's private key.
-  std::variant<Unset, FilePath, FileContents> pem_private_key;
+  absl::variant<Unset, FilePath, FileContents> pem_private_key;
 
   // Path to or contents of a .pem file containing the client's certificate chain.
-  std::variant<Unset, FilePath, FileContents> pem_cert_chain;
+  absl::variant<Unset, FilePath, FileContents> pem_cert_chain;
 };
 
 /**
@@ -61,7 +60,7 @@ struct OtlpGrpcExporterOptions
   // SSL encryption.
   std::string ssl_credentials_cacert_as_string = GetOtlpDefaultSslCertificateString();
   // The credentials to secure the connection.
-  std::optional<SSLCredentials> ssl_credentials;
+  absl::optional<SSLCredentials> ssl_credentials;
   // Timeout for grpc deadline
   std::chrono::system_clock::duration timeout = GetOtlpDefaultTimeout();
   // Additional HTTP headers