A filtering expression to specify restrictions on returned metrics. The expression is a sequence of terms. Each term applies a restriction to the returned metrics. Use this expression to restrict results to a specific time range. Currently we expect only one types of fields: * timestamp: This can be specified twice, once with a less than operator and once with a greater than operator. The timestamp restriction should result in one, contiguous, valid, timestamp range. Some examples of valid filters expressions: * Example 1: timestamp > "2012-04-23T18:25:43.511Z" timestamp < "2012-04-23T18:30:43.511Z" * Example 2: timestamp > "2012-04-23T18:25:43.511Z"