Add metrics related to tcmalloc acquiring and decommitting memory from system

From time to time tcmalloc acquires memory from the system using mmap, and returns it to the system using madvise. These can be expensive operations, and they hold a lock so they can create a bottleneck. There aren't adequate metrics to diagnose issues related to this, and to assess the effect of making changes such as enabling TCMALLOC_AGGRESSIVE_DECOMMIT. The following could be helpful:

• number of mmap operations to acquire memory
• number of bytes acquired
• cumulative time spent in such operations
• number of madvise oeprations to release memory
• number of bytes released
• cumulative time spent in such operations
• number of bytes allocated using decommitted memory (which kernel then must re-commit)