public com.google.protobuf.ByteString getFilterBytes() {
      java.lang.Object ref = filter_;
      if (ref instanceof String) {
        com.google.protobuf.ByteString b =
            com.google.protobuf.ByteString.copyFromUtf8((java.lang.String) ref);
        filter_ = b;
        return b;
      } else {
        return (com.google.protobuf.ByteString) ref;
      }
    }
<pre>
Optional. A filter against labels for the request.

By default, searches use prefix matching. To specify exact match, prepend
a plus symbol (`+`) to the search term.
Multiple terms are ANDed. Syntax:

*   `root:NAME_PREFIX` or `NAME_PREFIX`: Return traces where any root
    span starts with `NAME_PREFIX`.
*   `+root:NAME` or `+NAME`: Return traces where any root span's name is
    exactly `NAME`.
*   `span:NAME_PREFIX`: Return traces where any span starts with
    `NAME_PREFIX`.
*   `+span:NAME`: Return traces where any span's name is exactly
    `NAME`.
*   `latency:DURATION`: Return traces whose overall latency is
    greater or equal to than `DURATION`. Accepted units are nanoseconds
    (`ns`), milliseconds (`ms`), and seconds (`s`). Default is `ms`. For
    example, `latency:24ms` returns traces whose overall latency
    is greater than or equal to 24 milliseconds.
*   `label:LABEL_KEY`: Return all traces containing the specified
    label key (exact match, case-sensitive) regardless of the key:value
    pair's value (including empty values).
*   `LABEL_KEY:VALUE_PREFIX`: Return all traces containing the specified
    label key (exact match, case-sensitive) whose value starts with
    `VALUE_PREFIX`. Both a key and a value must be specified.
*   `+LABEL_KEY:VALUE`: Return all traces containing a key:value pair
    exactly matching the specified text. Both a key and a value must be
    specified.
*   `method:VALUE`: Equivalent to `/http/method:VALUE`.
*   `url:VALUE`: Equivalent to `/http/url:VALUE`.
</pre>

<code>string filter = 7 [(.google.api.field_behavior) = OPTIONAL];</code>

@return The bytes for filter.
java-trace/proto-google-cloud-trace-v1/src/main/java/com/google/devtools/cloudtrace/v1/ListTracesRequest.java