Dear @mariusz.panek ,
Please find attached api proxy that demonstrates same. You can just import this and test same. Create an API product and add your API proxy to product. You need to create couple of developers with custom attribute, apps for each developer to generate keys. Make sure you select the product while creating an developer app. Provided developer custom attributes screenshots.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<VerifyAPIKey async="false" continueOnError="false" enabled="true" name="Verify-API-Key-1">
<DisplayName>Verify API Key-1</DisplayName>
<Properties/>
<APIKey ref="request.queryparam.apikey"/>
</VerifyAPIKey>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Quota async="false" continueOnError="false" enabled="true" name="Quota-1">
<DisplayName>Dynamic Quota</DisplayName>
<Properties/>
<Allow count="1000" countRef="verifyapikey.Verify-API-Key-1.developer.allowed_quota"/>
<Interval ref="request.header.quota_count">1</Interval>
<TimeUnit ref="request.header.quota_timeout">day</TimeUnit>
<Identifier ref="client_id"/>
</Quota>
Cheers,
Anil Sagar