[#493] Refactor serving of prometheus and pprof services

Rename `util/profiler` package to `httputil` and refactor it:

  * simplify utility HTTP server;

  * make more generic server's parameters in order to remove `viper.Viper`
    dependency;

  * use single constructor for creating the pprof and prometheus servers;

  * replace `enabled` config value with empty-check of the network address.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/http/opts.go

@@ -0,0 +1,26 @@
+package httputil
+
+import (
+	"time"
+)
+
+// Option sets an optional parameter of Server.
+type Option func(*cfg)
+
+type cfg struct {
+	shutdownTimeout time.Duration
+}
+
+func defaultCfg() *cfg {
+	return &cfg{
+		shutdownTimeout: 15 * time.Second,
+	}
+}
+
+// WithShutdownTimeout returns option to set shutdown timeout
+// of the internal HTTP server.
+func WithShutdownTimeout(dur time.Duration) Option {
+	return func(c *cfg) {
+		c.shutdownTimeout = dur
+	}
+}