feat: create batch custom events sample

[BigBlackWolf]

Description

Fixes #

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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)
• [ ] 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[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [batch/create_with_custom_events.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/d3598d1ccdc104133c7082c0b9635130fae1ff5b/batch/create_with_custom_events.go#L17), tag `batch_custom_events`

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