Add v2 metric tags and metric volumes endpoints

[api-clients-generation-pipeline]

See DataDog/datadog-api-spec#836

[jirikuncar]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #769 (b006e90) into master (2862599) will decrease coverage by 0.12%.
The diff coverage is 20.27%.

Impacted Files	Coverage Δ
api/v2/datadog/model_metric_all_tags.go	0.00% <0.00%> (ø)
api/v2/datadog/model_metric_all_tags_attributes.go	0.00% <0.00%> (ø)
api/v2/datadog/model_metric_all_tags_response.go	0.00% <0.00%> (ø)
api/v2/datadog/model_metric_distinct_volume.go	0.00% <0.00%> (ø)
...datadog/model_metric_distinct_volume_attributes.go	0.00% <0.00%> (ø)
api/v2/datadog/model_metric_volumes_response.go	0.00% <0.00%> (ø)
...v2/datadog/model_metric_ingested_indexed_volume.go	13.55% <13.55%> (ø)
...pi/v2/datadog/model_metric_distinct_volume_type.go	14.28% <14.28%> (ø)
...model_metric_ingested_indexed_volume_attributes.go	14.28% <14.28%> (ø)
...tadog/model_metric_ingested_indexed_volume_type.go	21.42% <21.42%> (ø)
... and 22 more