feat(torsf): collect tor logs, select rendezvous method, count bytes (#683)

This diff contains significant improvements over the previous
implementation of the torsf experiment.

We add support for configuring different rendezvous methods after
the convo at https://github.com/ooni/probe/issues/2004. In doing
that, I've tried to use a terminology that is consistent with the
names being actually used by tor developers.

In terms of what to do next, this diff basically instruments
torsf to always rendezvous using domain fronting. Yet, it's also
possible to change the rendezvous method from the command line,
when using miniooni, which allows to experiment a bit more. In the
same vein, by default we use a persistent tor datadir, but it's
also possible to use a temporary datadir using the cmdline.

Here's how a generic invocation of `torsf` looks like:

```bash
./miniooni -O DisablePersistentDatadir=true \
           -O RendezvousMethod=amp \
           -O DisableProgress=true \
           torsf
```

(The default is `DisablePersistentDatadir=false` and
`RendezvousMethod=domain_fronting`.)

With this implementation, we can start measuring whether snowflake
and tor together can boostrap, which seems the most important thing
to focus on at the beginning. Understanding why the bootstrap most
often does not converge with a temporary datadir on Android devices
remains instead an open problem for now. (I'll also update the
relevant issues or create new issues after commit this.)

We also address some methodology improvements that were proposed
in https://github.com/ooni/probe/issues/1686. Namely:

1. we record the tor version;

2. we include the bootstrap percentage by reading the logs;

3. we set the anomaly key correctly;

4. we measure the bytes send and received (by `tor` not by `snowflake`, since
doing it for snowflake seems more complex at this stage).

What remains to be done is the possibility of including Snowflake
events into the measurement, which is not possible until the new
improvements at common/event in snowflake.git are included into a
tagged version of snowflake itself. (I'll make sure to mention
this aspect to @cohosh in https://github.com/ooni/probe/issues/2004.)

internal/tunnel/tunnel.go

@@ -74,6 +74,22 @@ var ErrEmptyTunnelDir = errors.New("TunnelDir is empty")
 // is not supported by this package.
 var ErrUnsupportedTunnelName = errors.New("unsupported tunnel name")
 
+// DebugInfo contains information useful to debug issues
+// when starting up a given tunnel fails.
+type DebugInfo struct {
+	// LogFilePath is the path to the log file, which MAY
+	// be empty in case we don't have a log file.
+	LogFilePath string
+
+	// Name is the name of the tunnel and will always
+	// be properly set by the code.
+	Name string
+
+	// Version is the tunnel version. This field MAY be
+	// empty if we don't know the version.
+	Version string
+}
+
 // Start starts a new tunnel by name or returns an error. We currently
 // support the following tunnels:
 //
@@ -94,7 +110,15 @@ var ErrUnsupportedTunnelName = errors.New("unsupported tunnel name")
 // The "fake" tunnel is a fake tunnel that just exposes a
 // SOCKS5 proxy and then connects directly to server. We use
 // this special kind of tunnel to implement tests.
-func Start(ctx context.Context, config *Config) (Tunnel, error) {
+//
+// The return value is a triple:
+//
+// 1. a valid Tunnel on success, nil on failure;
+//
+// 2. debugging information (both on success and failure);
+//
+// 3. nil on success, an error on failure.
+func Start(ctx context.Context, config *Config) (Tunnel, DebugInfo, error) {
 	switch config.Name {
 	case "fake":
 		return fakeStart(ctx, config)
@@ -103,6 +127,7 @@ func Start(ctx context.Context, config *Config) (Tunnel, error) {
 	case "tor":
 		return torStart(ctx, config)
 	default:
-		return nil, fmt.Errorf("%w: %s", ErrUnsupportedTunnelName, config.Name)
+		di := DebugInfo{}
+		return nil, di, fmt.Errorf("%w: %s", ErrUnsupportedTunnelName, config.Name)
 	}
 }