test: clean metrics before execution of metrics tests

Metrics is now enabled by default. We need to clear it state before
running the appropriate tests to make sure there is no data from
previous tests.

Related to #118

test/unit/metrics_test.lua

@@ -11,12 +11,17 @@ g.before_each(function()
     t.skip_if(not helpers.is_metrics_supported(),
               "metrics >= 0.11.0 is not installed")
     g.space = helpers.create_space_with_tree_index('memtx')
+    -- kill live tasks (it can still live after failed tests)
+    for _, t in ipairs(expirationd.tasks()) do
+        expirationd.kill(t)
+    end
+    -- disable and clean metrics by default
+    expirationd.cfg({metrics = false})
+    require('metrics').clear()
 end)
 
 local task = nil
 g.after_each(function(g)
-    expirationd.cfg({metrics = false}) -- reset metrics stats
-    require('metrics').clear()
     expirationd.cfg(g.default_cfg)
     g.space:drop()
     if task ~= nil then