Timer metric type

Add support for Timer metric type to the SDK.
Interface should be along the lines of:
```

void update(Duration duration);
T time(Callable event);
Closeable time();

```

Compare to http://metrics.dropwizard.io/3.1.0/apidocs/com/codahale/metrics/Timer.html

Could reuse code for this and https://issues.apache.org/jira/browse/BEAM-1613

Imported from Jira [BEAM-1699](https://issues.apache.org/jira/browse/BEAM-1699). Original Jira may contain additional context.
Reported by: aviemzur.

Start by locating the SDK's existing metric types, then compare the proposed interface with Dropwizard Timer and BEAM-1613. Done means the SDK supports the requested Timer operations; the issue names no specific files or tests, so those entry points and verification steps must be identified first.

Written by the indexing model from the issue text.