Optionally expose the type of metric via the Metrics API.

As more metrics are exposed we should allow for an optional flag to expose the underlying type to caller. Perhaps ```?type=true```.

This may allow the consumer of the metrics to make a more informed choice about what to do with these metrics.

For example:
```
"pipelines" : {
"main" : {
"events" : {
"duration_in_millis" : 59,
```
Could optionally be:
```
"pipelines" : {
"main" : {
"events" : {
"duration_in_millis" : {
"type" : "counter/long",
"value" : 59
},
```
In the above example, it is a counter backed by a long.

Other types may be ```gauge/text``` or ```gauge/numeric``` or ```gauge/boolean``` (or future) ```rate/long```.