chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/httptransport/bytecounter.go

@@ -0,0 +1,73 @@
+package httptransport
+
+import (
+	"io"
+	"net/http"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/bytecounter"
+)
+
+// ByteCountingTransport is a RoundTripper that counts bytes.
+type ByteCountingTransport struct {
+	RoundTripper
+	Counter *bytecounter.Counter
+}
+
+// RoundTrip implements RoundTripper.RoundTrip
+func (txp ByteCountingTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	if req.Body != nil {
+		req.Body = byteCountingBody{
+			ReadCloser: req.Body, Account: txp.Counter.CountBytesSent}
+	}
+	txp.estimateRequestMetadata(req)
+	resp, err := txp.RoundTripper.RoundTrip(req)
+	if err != nil {
+		return nil, err
+	}
+	txp.estimateResponseMetadata(resp)
+	resp.Body = byteCountingBody{
+		ReadCloser: resp.Body, Account: txp.Counter.CountBytesReceived}
+	return resp, nil
+}
+
+func (txp ByteCountingTransport) estimateRequestMetadata(req *http.Request) {
+	txp.Counter.CountBytesSent(len(req.Method))
+	txp.Counter.CountBytesSent(len(req.URL.String()))
+	for key, values := range req.Header {
+		for _, value := range values {
+			txp.Counter.CountBytesSent(len(key))
+			txp.Counter.CountBytesSent(len(": "))
+			txp.Counter.CountBytesSent(len(value))
+			txp.Counter.CountBytesSent(len("\r\n"))
+		}
+	}
+	txp.Counter.CountBytesSent(len("\r\n"))
+}
+
+func (txp ByteCountingTransport) estimateResponseMetadata(resp *http.Response) {
+	txp.Counter.CountBytesReceived(len(resp.Status))
+	for key, values := range resp.Header {
+		for _, value := range values {
+			txp.Counter.CountBytesReceived(len(key))
+			txp.Counter.CountBytesReceived(len(": "))
+			txp.Counter.CountBytesReceived(len(value))
+			txp.Counter.CountBytesReceived(len("\r\n"))
+		}
+	}
+	txp.Counter.CountBytesReceived(len("\r\n"))
+}
+
+type byteCountingBody struct {
+	io.ReadCloser
+	Account func(int)
+}
+
+func (r byteCountingBody) Read(p []byte) (int, error) {
+	count, err := r.ReadCloser.Read(p)
+	if count > 0 {
+		r.Account(count)
+	}
+	return count, err
+}
+
+var _ RoundTripper = ByteCountingTransport{}