State-db I/o metrics (#4562)

* add usage mod to sp-state-machine

* State usage tracking finalized.

* fix spaces

* add license preamble

* imporove output

* review suggestions

* update naming

* merge fixes

* Update client/db/src/light.rs

Co-Authored-By: Gavin Wood <gavin@parity.io>

Co-authored-by: Gavin Wood <github@gavwood.com>

substrate/primitives/state-machine/src/stats.rs

@@ -0,0 +1,62 @@
+// Copyright 2017-2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Usage statistics for state db
+
+use std::time::{Instant, Duration};
+
+/// Measured count of operations and total bytes.
+#[derive(Clone, Debug, Default)]
+pub struct UsageUnit {
+    /// Number of operations.
+    pub ops: u64,
+    /// Number of bytes.
+    pub bytes: u64,
+}
+
+/// Usage statistics for state backend.
+#[derive(Clone, Debug)]
+pub struct UsageInfo {
+    /// Read statistics (total).
+    pub reads: UsageUnit,
+    /// Write statistics.
+    pub writes: UsageUnit,
+    /// Cache read statistics.
+    pub cache_reads: UsageUnit,
+    /// Memory used.
+    pub memory: usize,
+
+    /// Moment at which current statistics has been started being collected.
+    pub started: Instant,
+    /// Timespan of the statistics.
+    pub span: Duration,
+}
+
+impl UsageInfo {
+    /// Empty statistics.
+    ///
+    /// Means no data was collected.
+    pub fn empty() -> Self {
+        Self {
+            reads: UsageUnit::default(),
+            writes: UsageUnit::default(),
+            cache_reads: UsageUnit::default(),
+            memory: 0,
+            started: Instant::now(),
+            span: Default::default(),
+        }
+    }
+}