[doc][develop] Add resource_monitor_plugin documentation

docs/01_nodeos/03_plugins/resource_monitor_plugin/index.md

@@ -0,0 +1,71 @@
+
+## Overview
+
+The `resource_monitor_plugin` monitors space usage in the computing system where `nodeos` is running. Specifically, every `resource-monitor-interval-seconds` seconds,
+it measures the individual space used by each of the file systems mounted
+by `data-dir`, `state-dir`, `blocks-log-dir`, `snapshots-dir`,
+`state-history-dir`, and `trace-dir`.
+When space usage in any of the monitored file system is within `5%` of the threshold
+specified by `resource-monitor-space-threshold`, a warning containing the file system
+path and percentage of space has used is printed out.
+When space usage exceedes the threshold,
+if `resource-monitor-not-shutdown-on-threshold-exceeded` is not set,
+`nodeos` gracefully shuts down; if `resource-monitor-not-shutdown-on-threshold-exceeded` is set, `nodeos` prints out warnings perioidically
+until space usage goes under the threshold.
+
+`resource_monitor_plugin` is always loaded.
+## Usage
+
+```console
+# config.ini
+plugin = eosio::resource_monitor_plugin
+[options]
+```
+```sh
+# command-line
+nodeos ... --plugin eosio::resource_monitor_plugin [options]
+```
+
+## Configuration Options
+
+These can be specified from both the `nodeos` command-line or the `config.ini` file:
+
+```console
+Config Options for eosio::resource_monitor_plugin:
+
+  --resource-monitor-interval-seconds arg (=2)
+                                        Time in seconds between two consecutive
+                                        checks of space usage. Should be
+                                        between 1 and 300.
+  --resource-monitor-space-threshold arg (=90)
+                                        Threshold in terms of percentage of used
+                                        space vs total space. If the used space is
+                                        within `%5` of the threshold, a warning is
+                                        generated. If used space is above the
+                                        threshold and `resource-monitor-not-shutdown-on-threshold-exceeded`
+                                        is enabled, a graceful shutdown is initiated;
+                                        otherwise a warning will be continuously
+                                        printed out.
+                                        The value should be between 6 and 99.
+  --resource-monitor-not-shutdown-on-threshold-exceeded
+                                        A switch used to indicate `nodeos` will "not"
+                                        shutdown when threshold is exceeded. When not
+                                        set, `nodeos` will shutdown.
+  --resource-monitor-warning-interval arg (=30)
+                                        Number of monitor intervals between
+                                        which a warning is displayed.
+                                        For example,
+                                        if `resource-monitor-warning-interval`
+                                        is set to `10` and 
+                                        `resource-monitor-interval-seconds` is 2, 
+                                        a warning will be displayed every 20 seconds,
+                                        even though the space usage is checked every
+                                        2 seconds.
+                                        This is used to throttle the number of
+                                        warnings in the `nodeos` log file.
+                                        Should be between 1 and 450.
+```
+
+## Plugin Dependencies
+
+* None