The client sends a x-elastic-client-meta header by default. This header is used to collect meta data about the versions of the components used by the client. For instance, a value of x-elastic-client-meta can be es=8.0.0-s,php=8.0.0,t=8.0.0-s,a=0,gu=7.4.2, where each value is the version of `es=Elasticsearch, t is the transport version (same of client), a is asyncronouts (0=false by default) and gu=Guzzle.

If you would like to disable it you can use the setElasticMetaHeader() method, as follows:

$client = Elasticsearch\ClientBuilder::create()
    ->setElasticMetaHeader(false)
    ->build();