admin/info: Add HTTPStats value as part of serverInfo() struct. (#4049)

Remove our counter implementation instead use atomic external
package which supports more types and methods.

vendor/go.uber.org/atomic/README.md

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+# atomic [![GoDoc][doc-img]][doc] [![Build Status][ci-img]][ci] [![Coverage Status][cov-img]][cov]
+
+Simple wrappers for primitive types to enforce atomic access.
+
+## Installation
+`go get -u go.uber.org/atomic`
+
+## Usage
+The standard library's `sync/atomic` is powerful, but it's easy to forget which
+variables must be accessed atomically. `go.uber.org/atomic` preserves all the
+functionality of the standard library, but wraps the primitive types to
+provide a safer, more convenient API.
+
+```go
+var atom atomic.Uint32
+atom.Store(42)
+atom.Sub(2)
+atom.CAS(40, 11)
+```
+
+See the [documentation][doc] for a complete API specification.
+
+## Development Status
+Stable.
+
+<hr>
+Released under the [MIT License](LICENSE.txt).
+
+[doc-img]: https://godoc.org/github.com/uber-go/atomic?status.svg
+[doc]: https://godoc.org/go.uber.org/atomic
+[ci-img]: https://travis-ci.org/uber-go/atomic.svg?branch=master
+[ci]: https://travis-ci.org/uber-go/atomic
+[cov-img]: https://coveralls.io/repos/github/uber-go/atomic/badge.svg?branch=master
+[cov]: https://coveralls.io/github/uber-go/atomic?branch=master