Yaml parsing refactor (#299)

* Use struct instead of interface{} when parsing query user

* Use MappingOptions

* Split function to be more testable

* Rename function to parseUserQueries

* Start to add test about query parsing

cmd/postgres_exporter/tests/user_queries_ok.yaml

@@ -0,0 +1,23 @@
+pg_locks_mode:
+  query: "WITH q_locks AS (select * from pg_locks where pid != pg_backend_pid() and database = (select oid from pg_database where datname = current_database())) SELECT (select current_database()) as datname,
+  lockmodes AS tag_lockmode, coalesce((select count(*) FROM q_locks WHERE mode = lockmodes), 0) AS count FROM
+  unnest('{AccessShareLock, ExclusiveLock, RowShareLock, RowExclusiveLock, ShareLock, ShareRowExclusiveLock,  AccessExclusiveLock, ShareUpdateExclusiveLock}'::text[]) lockmodes;"
+  metrics:
+    - datname:
+        usage: "LABEL"
+        description: "Database name"
+    - tag_lockmode:
+        usage: "LABEL"
+        description: "Lock type"
+    - count:
+        usage: "GAUGE"
+        description: "Number of lock"
+pg_wal:
+  query: "select current_database() as datname, case when pg_is_in_recovery() = false then pg_xlog_location_diff(pg_current_xlog_location(), '0/0')::int8 else pg_xlog_location_diff(pg_last_xlog_replay_location(), '0/0')::int8 end as xlog_location_b;"
+  metrics:
+    - datname:
+        usage: "LABEL"
+        description: "Database name"
+    - xlog_location_b:
+        usage: "COUNTER"
+        description: "current transaction log write location"