Must be a valid JSON object and cannot contain functions, undefined, Infinity and NaN as
values, otherwise they will be replaced with null. The fields need to be serializable
using JSON.stringify. All keys must be prefixed with "session_properties.*" and must contain only
lower-case characters, numbers, underscores or dots. Each dot must be followed by a lower-case
character. Each underscore must be followed by a lower-case character or number.
Sends a sessionPropertyEvent.