[receiver/splunkhec] Avoid a memory leak

[atoulme]

Description

Avoid a memory leak by not recording obsreports if the receiver is used for both logs and metrics.

Link to tracking issue

Fixes #35294

[dmitryax]

I don’t think this should be fixed this way. It's going to be very confusing why it only works if enabled on one type of pipeline. It’s currently a mishmash of logs/metrics reporting if both are enabled. This is likely what is causing all the problems. Instead of spreading around obsrecv.EndLogsOp and obsrecv.EndMetricsOp, I think we need to wrap only metricsConsumer.ConsumeMetrics and logsConsumer.ConsumeLogs, same as done in otlp receiver

	ctx = r.obsreport.StartMetricsOp(ctx)
	err := r.metricsConsumer.ConsumeMetrics(ctx, md)
	r.obsreport.EndMetricsOp(ctx, dataFormat, dataPointCount, err)

And, instrumentation of the HTTP server should be done separately if it's not currently enabled

[atoulme]

Closing in favor of #36146