Don't pass a reused bytes.Buffer to the ES client

[vikmik]

See comments.

Directly passing a reusable bytes.Buffer is unsafe because of potential race condition on error scenarios introduced by the underlying RoundTripper implementation.

See:
https://pkg.go.dev/net/http#RoundTripper

// RoundTrip must always close the body, including on errors,
// but depending on the implementation may do so in a separate
// goroutine even after RoundTrip returns. This means that
// callers wanting to reuse the body for subsequent requests
// must arrange to wait for the Close call before doing so.

We don't need a full copy of the buffer underlying slice since the sending goroutine is only reading from the buffer. We just need to make sure that bytes.Buffer other member variables aren't accessed and modified concurrently by different goroutines.

The call to bytes.NewBuffer(b.buf.Bytes()) mimics what's done internally by the net/http library (and other implementations) on every retry: https://github.com/golang/go/blob/08e73e61521d7b83198407211aa232ed4f572f18/src/net/http/request.go#L929 - that is, taking the underlying byte slice and wrapping it in a brand new bytes.Buffer prior to any retry.

Note that GetBody is generally not invoked on the first try of a request, which means the original request body is directly read from. Prior to this PR, b.buf could be directly read from even after the call to req.Do returned.

Related: golang/go#51907