[RQ-57] Usage Billable Summary response

[api-clients-generation-pipeline]

See DataDog/datadog-api-spec#460

[nmuesch]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #367 into master will decrease coverage by 0.32%.
The diff coverage is 35.23%.

Impacted Files	Coverage Δ	Complexity Δ
...dog/api/v2/client/model/LogsListResponseLinks.java	19.04% <19.04%> (ø)	3.00 <3.00> (?)
...adog/api/v2/client/model/LogsListResponseMeta.java	19.04% <19.04%> (ø)	3.00 <3.00> (?)
.../api/v2/client/model/LogsListResponseMetaPage.java	19.04% <19.04%> (ø)	3.00 <3.00> (?)
.../v1/client/model/UsageBillableSummaryResponse.java	19.23% <19.23%> (ø)	3.00 <3.00> (?)
.../api/v1/client/model/UsageBillableSummaryKeys.java	21.42% <21.42%> (ø)	16.00 <16.00> (?)
...com/datadog/api/v2/client/model/LogAttributes.java	21.91% <21.91%> (ø)	8.00 <8.00> (?)
...dog/api/v2/client/model/LogsListRequestFilter.java	25.53% <25.53%> (ø)	8.00 <8.00> (?)
.../datadog/api/v2/client/model/LogsListResponse.java	27.50% <27.50%> (ø)	7.00 <7.00> (?)
...tadog/api/v2/client/model/LogsListRequestPage.java	27.58% <27.58%> (ø)	5.00 <5.00> (?)
...main/java/com/datadog/api/v2/client/model/Log.java	27.77% <27.77%> (ø)	6.00 <6.00> (?)
... and 29 more