feat(oohelperd): add prometheus metrics (#897)

Closes https://github.com/ooni/probe/issues/2183

While there, avoid exposing nil values for optional fields of the
THResponse struct (i.e., "ip_info" and "tls_handshake").

While there, fix `measurexlite`'s `OperationLogger` test
and make it deterministic rather than racy.

internal/model/th.go

@@ -78,8 +78,8 @@ const (
 // THResponse is the response from the control service.
 type THResponse struct {
 	TCPConnect   map[string]THTCPConnectResult   `json:"tcp_connect"`
-	TLSHandshake map[string]THTLSHandshakeResult `json:"tls_handshake"`
+	TLSHandshake map[string]THTLSHandshakeResult `json:"tls_handshake,omitempty"`
 	HTTPRequest  THHTTPRequestResult             `json:"http_request"`
 	DNS          THDNSResult                     `json:"dns"`
-	IPInfo       map[string]*THIPInfo            `json:"ip_info"`
+	IPInfo       map[string]*THIPInfo            `json:"ip_info,omitempty"`
 }