chore: release 1.18.0

[yoshi-code-bot]

🤖 I have created a release *beep* *boop*

1.18.0 (2021-05-03)

Features

• billing/budgets: Added support for configurable budget time period. fix: Updated some documentation links. (83b1b3b)
• billing/budgets: Added support for configurable budget time period. fix: Updated some documentation links. (83b1b3b)
• cloudbuild/apiv1: Add fields for Pub/Sub triggers (8b4adbf)
• dataproc: update the Dataproc V1 API client library (9a459d5)
• dialogflow/cx: support setting current_page to resume sessions; expose transition_route_groups in flows and language_code in webhook (9a459d5)
• dialogflow/cx: support setting current_page to resume sessions; expose transition_route_groups in flows and language_code in webhook (9a459d5)
• documentai: add confidence field to the PageAnchor.PageRef in document.proto. (d089dda)
• documentai: add confidence field to the PageAnchor.PageRef in document.proto. (07fdcd1)
• internal/gensnippets: add license header and region tags (#3924) (e9ff7a0)
• internal/gensnippets: initial commit (#3922) (3fabef0)
• internal: auto-generate snippets (#3949) (b70e0fc)
• internal: generate region tags for snippets (#3962) (ef2b90e)
• security/privateca: start generating apiv1 (#4023) (08aa83a)
• speech: add webm opus support. (d089dda)
• speech: Support for spoken punctuation and spoken emojis. (9a459d5)

Bug Fixes

• binaryauthorization: add Java options to Binaryauthorization protos (9a459d5)
• internal/gapicgen: use correct region tags for gensnippets (#4022) (8ccd689)
• internal/gensnippets: run goimports (#3931) (10050f0)
• internal: append a new line to comply with go fmt (#4028) (a297278)
• internal: make sure formatting is run on snippets (#4039) (130dfc5), refs #4037
• metastore: increase metastore lro polling timeouts (83b1b3b)

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