ElasticSearch exporter does not give the possibility to add custom headers (auth headers for instance)

[MaxLeb]

Describe your environment

Using a manually built version of OpenTelemetry 1.16.1 on macOS with :

-DWITH_EXAMPLES=OFF
-DWITH_OTLP_HTTP=ON
-DBUILD_TESTING=OFF
-DWITH_OTLP_GRPC=OFF
-DWITH_LOGS_PREVIEW=ON
-DOPENTELEMETRY_INSTALL=ON
-DBUILD_SHARED_LIBS=OFF
-DWITH_ASYNC_EXPORT_PREVIEW=ON
-DCMAKE_CXX_STANDARD=17
-DWITH_ELASTICSEARCH=ON

Steps to reproduce

The usage is pretty simple:

First an init of the exporter with a single log record processor:

opentelemetry::exporter::logs::ElasticsearchExporterOptions opts(
,
9200,
"logs",
30,
true);

auto elkExporter = std::make_uniqueopentelemetry::exporter::logs::ElasticsearchLogRecordExporter(opts);

auto elkProcessor = opentelemetry::sdk::logs::SimpleLogRecordProcessorFactory::Create(std::move(elkExporter));

std::shared_ptropentelemetry::logs::LoggerProvider provider = opentelemetry::sdk::logs::LoggerProviderFactory::Create(
std::move(elkProcessor), resource);

opentelemetry::logs::Provider::SetLoggerProvider(provider);

m_Logger = provider->GetLogger("MyLogger", SOURCE_NAME);
Then, when logs needs to be sent a call to m_Logger.EmitLogRecord(xxx)

What is the expected behavior?
Since the ELK endpoint requires a basic auth, the call is rejected with a 401 error as we cannot set any custom headers to the request.

What is the actual behavior?
Add possibility to add custom headers (probably more of a feature request than an actual bug).

[MaxLeb]

Proposed patch :
0001-EXPORTER-Add-possibility-to-give-custom-HttpHeaders-.patch