Failed to build opentelemetry-cpp with OTLP HTTP exporter

[thangdinh1298]

Describe your environment
OS: Debian 10
Release version: opentelemetry-cpp v1.1.0
Compiler: g++ 8.3.0

Steps to reproduce

cmake ../ -DWITH_OTLP=ON -DWITH_OTLP_HTTP=ON -DBUILD_TESTING=OFF
make -j

What is the expected behavior?
Builds successfully

What is the actual behavior?
The build failed with the following error

opentelemetry-cpp/exporters/otlp/src/otlp_http_client.cc:292:40: error: cannot convert 'const string' {aka 'const std::__cxx11::basic_string'} to 'const unsigned char*'
google::protobuf::Base64Escape(bytes, &base64_value);

opentelemetry-cpp/exporters/otlp/src/otlp_http_client.cc:31:58: note: initializing argument 1 of 'int google::protobuf::Base64Escape(const unsigned char*, int, char*, int)'
LIBPROTOBUF_EXPORT int Base64Escape(const unsigned char *src, int slen, char *dest, int szdest);

Aditional Context
It seems that the function Base64Escape was being called with the incorrect set of arguments. It was called with bytes, which is an std::string instead of a const unsigned char *.

[esigo]

Thanks for reporting. We should use a different overload here.