pmproxy: fix memory corruption due to missing metric_table entry for "search"
pmproxy: fix memory corruption due to missing metric_table entry for "search"

Resolves: https://github.com/performancecopilot/pcp/issues/1003

In pmproxy server.c, the metrics_table array was missing an entry
for METRICS_SEARCH. Valgrind traces showed invalid writes during
initialization and consequential corruption in other data structures
and (as shown in qa/1600) eventually segfaulting.

With this fix, qa/1600 is now passing again and pmproxy is behaving.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp