1. A method for processing a dynamically changing set of data streams using a data stream language program, the method comprising: storing, by an instrumentation analysis system, metadata describing a plurality of data streams; receiving a data stream language program comprising a set of instructions specified using a data stream language, the instructions comprising a find block associated with a search expression, the search expression based on metadata attributes associated with data streams; evaluating the search expression to identify a set of data streams from the plurality of data streams conforming to the search expression; and repeatedly executing the data stream language program, the execution comprising: receiving data values from each data stream of the identified set of data streams, executing each block of the data stream language program, generating one or more result data values based on the execution, the result values corresponding to result data streams generated by the data stream language program, and storing the one or more result data values.