rename gain_adjustment_display to gain_adjustment_metering (#444)

* adding ext/draft/gain-reduction.h

* add documentation, rename accessor function

* document that the returned value represents dynamic gain reduction/expansion before make-up gain is applied

* clarify what the returned value represents more precisely

* rename the extension to gain-adjustment-display

* rename gain-adjustment-display to gain-adjustment-metering

Author: schwaaa

include/clap/ext/draft/gain-adjustment-metering.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "../../plugin.h"
+
+// This extension lets the plugin report the current gain adjustment
+// (typically, gain reduction) to the host.
+
+static CLAP_CONSTEXPR const char CLAP_EXT_GAIN_ADJUSTMENT_METERING[] = "clap.gain-adjustment-metering/0";
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct clap_plugin_gain_adjustment_metering {
+  // Returns the current gain adjustment in dB. The value is intended
+  // for informational display, for example in a host meter or tooltip.
+  // The returned value represents the gain adjustment that the plugin
+  // applied to the last sample in the most recently processed block.
+  //
+  // The returned value is in dB. Zero means the plugin is applying no gain
+  // reduction, or is not processing. A negative value means the plugin is
+  // applying gain reduction, as with a compressor or limiter. A positive
+  // value means the plugin is adding gain, as with an expander. The value
+  // represents the dynamic gain reduction or expansion applied by the
+  // plugin, before any make-up gain or other adjustment. A single value is
+  // returned for all audio channels.
+  //
+  // [audio-thread]
+  double(CLAP_ABI *get)(const clap_plugin_t *plugin);
+} clap_plugin_gain_adjustment_metering_t;
+
+#ifdef __cplusplus
+}
+#endif