go: Update all dependencies, imports

[minor-fixes]

This change updates dependencies in go.mod to their latest versions, and import paths to the latest supported package/major version. Keeping dependencies up-to-date helps minimize dependency issues in downstream projects that track dependencies closer to latest (especially in the face of Google API package migrations)

This includes:

• Using cloud.google.com/go/monitoring/apiv3/v2 instead of the v1 API cloud.google.com/go/monitoring/apiv3 - this doesn't appear to break existing code, according to the build/unit tests
• Using github.com/hashicorp/go-metrics in place of github.com/armon/go-metrics, now that the latter URL redirects to the former. There is no release of github.com/hashicorp/go-metrics that has that name in go.mod, so the most recent master commit is used instead.
• Using protobuf libraries from google.golang.org/protobuf instead of github.com/golang/protobuf - minor source changes are made to use the more succinct timestamppb package
• Updating all dependencies via go get -u ./...
• Now that code depends on stdlib features in 1.17, marking the go.mod file as such, and migrating it via go mod tidy -compat=1.17
• Updating Github Actions checks to test Go versions 1.17-1.20, inclusive

Tested:

• Builds and tests pass: go build ./... && go test ./...
• Actions pass locally via gh act

Not tested:

• Whether application still works at runtime

[maxb]

• There is no release of github.com/hashicorp/go-metrics that has that name in go.mod

Releases have subsequently been made.