Support filtering by elasticsearch spans

[aspectom]

Currently, we allow to filter by specific databases/orm like mongo/sequelize.

We want to enable this same behavior on elasticsearch, so users could filter by elasticsearch spans easily.

This is an example of usage in sequelize

const sequelizeActivities = spans.sequelize();
expect(sequelizeActivities.length).toBe(1);
expect(sequelizeActivities.first.dbOperation).toBe("SELECT");
expect(Array.isArray(JSON.parse(sequelizeActivities.first.dbResponse))).toBe(true);

We would want to enable something like this:

const esSpans = spans.elasticsearch();
expect(esSpans.length).toBe(1);

It means to implement this filter in SpansRepository class. See how it's implemented for mongo/sequelize etc.