Benchmark collectives refactoring and fixes (#32)

* Rename get_metrics_helper to extract_metadata This commit renames the helper function `get_metrics_helper` to `extract_metadata` to better reflect its purpose of extracting metadata from the benchmark parameters. * Extract common collectives benchmarking logic into a single function Introduces a helper function "benchmark_collective" to encapsulate the common logic for running DCN and ICI benchmarks, across all the different benchmark functions. * Extract collectives metrics statistics generation into helper function Introduces a helper function "generate metrics statistics" to encapsulate the common logic to generate and print statistics from a metrics list. * Move ici_average_time_ms_list processing into benchmark_collectives - This specific metric is only produced by collective benchmarks - Also generalized the serializing of nested data structures into json strings instead of serializing only ici_average_time_ms_list * Exclude ici/dci_average_time_ms_list from collectives metadata This are no ici/dci_average_time_ms symbols in all calculate_metrics functions. So the correct symbol to exclude when generating metadata should be ici/dci_average_time_ms_list. * Remove all mentions to average_time The concept of average time isn't really applicable in all these situations since the time generated here comes from a single measurement rather than the average of multiple measurements. * Remove unused create_mesh return values

Author: Boon-Jun