This requires bumping https://github.com/mitchellh/go-testing-interface.
For this new version, we have to create a wrapper to convert
the stdlib `testing.TB` interface to the
`mitchellh/go-testing-interface` `T` interface, since it uses
`Parallel()` now, which is not supported by `testing.TB`. This had to be
added to a new package, `benchhelpers`, to avoid a circular dependency
in `testhelpers`.
We also have to *unbump* https://github.com/armon/go-metrics since
updating it breaks our usage of
https://github.com/google/go-metrics-stackdriver
I verified that the new `pkiCert` template function works with agent
injection using annotations like:
```yaml
vault.hashicorp.com/agent-inject-secret-sample.crt: "pki/issue/example-dot-com"
vault.hashicorp.com/agent-inject-template-sample.crt: |
{{ pkiCert "pki/issue/example-dot-com" "common_name=foo.example.com" "ttl=1h" }}
```
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