feat(datastore): Datastore query profiling samples

[bhshkh]

Description

This PR adds samples for query profiling. This PR will be merged after googleapis/google-cloud-go#9200

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
  Will re-trigger once feat(datastore): Add query profiling googleapis/google-cloud-go#9200 has been merged. Passes locally.
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• datastore/snippets/snippet_test.go:780, tag datastore_query_explain_entity
• datastore/snippets/snippet_test.go:805, tag datastore_query_explain_analyze_entity
• datastore/snippets/snippet_test.go:851, tag datastore_query_explain_aggregation
• datastore/snippets/snippet_test.go:876, tag datastore_query_explain_analyze_aggregation

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[bhshkh]

Do not merge until release freeze ends in mid April

[danieljbruce]

Do we need runAggregationQuery examples as well?

[bhshkh]

Do we need runAggregationQuery examples as well?

Added

[gkevinzheng]

LGTM