private val serverWriter =
    WebSocketWriter(
      isClient = false,
      sink = data,
      random = random,
      perMessageDeflate = false,
      noContextTakeover = false,
      minimumDeflateSize = 0L,
    )
  private val clientWriter =
    WebSocketWriter(
      isClient = true,
      sink = data,
      random = random,
      perMessageDeflate = false,
      noContextTakeover = false,
      minimumDeflateSize = 0L,

The `EventListener.Factory` also makes it possible to limit metrics to a subset of calls. This one captures metrics on a random 10%:

```java
class MetricsEventListener extends EventListener {
  private static final Factory FACTORY = new Factory() {
    @Override public EventListener create(Call call) {
      if (Math.random() < 0.10) {
        return new MetricsEventListener(call);
      } else {
        return EventListener.NONE;

      connection: Connection,
    ) {
      if (random != null) {
        val sslSocket = connection.socket() as SSLSocket
        val session = sslSocket.session

        val masterSecretHex =
          session.masterSecret?.encoded?.toByteString()
            ?.hex()

        if (masterSecretHex != null) {
          val keyLog = "CLIENT_RANDOM $random $masterSecretHex"

          if (verbose) {

        }
      }
    val webSocket =
      RealWebSocket(
        taskRunner = taskRunner,
        originalRequest = fancyRequest,
        listener = webSocketResponse.webSocketListener!!,
        random = SecureRandom(),
        pingIntervalMillis = 0,
        extensions = WebSocketExtensions.parse(webSocketResponse.headers),
        // Compress all messages if compression is enabled.
        minimumDeflateSize = 0L,

    val s = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
    for (i in s.indices) {
      assertRoundTrip(s.substring(0, i).encodeUtf8())
    }
    val random = Random(123456789L)
    val buf = ByteArray(4096)
    random.nextBytes(buf)
    assertRoundTrip(buf.toByteString())
  }

  private fun assertRoundTrip(data: ByteString) {
    val encodeBuffer = Buffer()
    encode(data, encodeBuffer)

 *
 * This class is not thread safe.
 *
 * [rfc_6455]: http://tools.ietf.org/html/rfc6455
 */
class WebSocketWriter(
  private val isClient: Boolean,
  val sink: BufferedSink,
  val random: Random,
  private val perMessageDeflate: Boolean,
  private val noContextTakeover: Boolean,
  private val minimumDeflateSize: Long,
) : Closeable {
  /** This holds outbound data for compression and masking. */

/**
 * Read and write a target file. Unlike Okio's built-in `Okio.source(java.io.File file)` and `Okio.sink(java.io.File file)`
 * this class offers:
 *
 *  * **Read/write:** read and write using the same operator.
 *  * **Random access:** access any position within the file.
 *  * **Shared channels:** read and write a file channel that's shared between
 * multiple operators. Note that although the underlying [FileChannel] may be shared,

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     * Set this certificate's common name (CN). Historically this held the hostname of TLS
     * certificate, but that practice was deprecated by [RFC 2818][rfc_2818] and replaced with
     * [addSubjectAlternativeName]. If unset a random string will be used.
     *
     * [rfc_2818]: https://tools.ietf.org/html/rfc2818
     */
    fun commonName(cn: String) =
      apply {
        this.commonName = cn
      }