Add Prometheus with initial metrics

[jeffswensen]

very much still a draft.

so far:

• adds prometheus w/ default go metrics
• adds searcher_started_total - counter incremented for each Searcher created
• adds search_duration_seconds - histogram for the search API endpoint (tracks duration, count)

still to do:

• disk usage
• errors
• update docs w/ instructions

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

The PR fulfills these requirements:

• All tests are passing?
• New/updated tests are included?
• If any static assets have been updated, has ui/bindata.go been regenerated?
• Are there doc blocks for functions that I updated/created?

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: