mgr: add new API's to fetch latest perf counter values

mgr: add new API's to fetch latest perf counter values

New API's to fetch the latest values of performance counters have been
added. These API's support fetching the values for labeled performance
counters.

We also make changes to `insert` and `create` API of
perf_counters_key to remove any empty labels. This change helps us
construct perf_counters_key when the key labels are constructed
dynamically.

Signed-off-by: Naveen Naidu <naveen.naidu@ibm.com>