**WIP**:

* has a TODO about adding name of failing predicate query to metric set
* defines the TTL field but does not implement it
* lacks test cover

Add "predicate queries" to decide whether a given metric query should
be scraped at runtime. Fixes Vonng#41.

These can be used to express complex conditions that may change over
time, such as "collect this metric set if the foo extension is installed
and is at least version 2.1", "collect this metric only when running on
(some specific vendor postgres flavour)" or "only try to collect this if
this specific table exists and has this specific column".

This helps with maintaining a single configuration to support multiple
postgres versions, vendor flavours, access privilege levels, installed
extensions and extension versions, etc. E.g. you can selectively query
`pg_stat_statements` only if the extension is installed in the current
DB.

The new syntax is added at query level in the configuration and looks
like this:

  rowcount_in_foo_table:
    predicate_queries:
      - name: foo_table exists
        predicate_query: |
          SELECT EXISTS (SELECT 1 FROM information_schema.tables WHERE table_schema = 'public' AND table_name = 'foo')
    query: |
      SELECT count(1) FROM public.foo;