ProfilerOutputStats: tweak stat key normalization

ProfilerOutputStats: tweak stat key normalization

When we convert a profiler entry to a statsd metric key, we replace
nonalphabetic characters with slashes. When part of the key is a file path with
a leading or trailing slash, this can result in keys that look like:
'MediaWiki._srv_mediawiki_foo_.bar', which is a bit ugly. This patch trims
leading and trailing underscores from each dot-separated key segment.

Change-Id: I71172b956f5a6120a75981a15dedf372d3893b88