allow adding data from multiple metrics

Here is an example request body,

[
  {
    "tenantId": "rhq-metrics",
    "name": "M10",
    "data": [
      {"timestamp": 1416276133964, "value": 2.1},
      {"timestamp": 1416276133965, "value": 2.2},
      {"timestamp": 14162761339646, "value": 2.3}
    ]
  },
  {
    "tenantId": "rhq-metrics",
    "name": "M11",
    "attributes": {"env": "test", "units": "MB"},
    "data": [
      {"timestamp": 1416276133964, "value": 2.1},
      {"timestamp": 1416276133965, "value": 2.2},
      {"timestamp": 14162761339646, "value": 2.3}
    ]
  },
  {
    "tenantId": "rhq-metrics",
    "name": "M12",
    "attributes": {"env": "test", "units": "bytes"}
  }
]

The object for metric M10 only includes data. The object for M11 contains data
and attributes. The object for M12 contains only attributes. This demonstrates
the three different types of input for numeric data. We may also want to allow
for tags.