Add limit parameter for get usage top average metrics

[api-clients-generation-pipeline]

See DataDog/datadog-api-spec#682

[jirikuncar]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #586 (0cc584b) into master (0134ebe) will increase coverage by 29.13%.
The diff coverage is 0.00%.

Impacted Files	Coverage Δ
api/v1/datadog/api_usage_metering.go	61.74% <0.00%> (+59.67%)	⬆️
...pi/v1/datadog/model_usage_billable_summary_keys.go	0.40% <0.00%> (+0.40%)	⬆️
...1/datadog/model_synthetics_api_test_result_data.go	0.68% <0.00%> (+0.68%)	⬆️
api/v2/datadog/model_log_attributes.go	0.93% <0.00%> (+0.93%)	⬆️
api/v2/datadog/model_logs_response_metadata.go	1.23% <0.00%> (+1.23%)	⬆️
...pi/v2/datadog/model_incident_team_response_data.go	1.38% <0.00%> (+1.38%)	⬆️
api/v2/datadog/model_role_update_response_data.go	1.47% <0.00%> (+1.47%)	⬆️
...el_synthetics_trigger_ci_tests_response_results.go	1.47% <0.00%> (+1.47%)	⬆️
api/v1/datadog/model_ip_ranges.go	1.50% <0.00%> (+1.50%)	⬆️
api/v2/datadog/model_incident_response_data.go	1.51% <0.00%> (+1.51%)	⬆️
... and 511 more