devex: add log tracing feature

simply set `trace_logs` to `true` in your config and you can easily
figure out where logs are coming from.

src/backend/doc/log_config.md

@@ -21,6 +21,14 @@ Sometimes "enabling" a log means moving its log level from `debug` to `info`.
 - `http`: http requests
 - `fsentries-not-found`: information about files that were stat'd but weren't there
 
+#### Other log options
+
+- Setting `log_upcoming_alarms` to `true` will log alarms before they are created.
+  This would be useful if AlarmService itself is failing.
+- Setting `trace_logs` to `true` will display a stack trace below every log message.
+  This can be useful if you don't know where a particular log is coming from and
+  want to track it down.
+
 #### Service-level log configuration
 
 Services can be configured to change their logging behavior. Services will have one of

src/backend/src/modules/core/LogService.js

@@ -607,6 +607,10 @@ class LogService extends BaseService {
         try {
             // skip messages that are above the output level
             if ( log_lvl.ordinal > this.output_lvl ) return;
+            
+            if ( this.config.trace_logs ) {
+                fields.stack = (new Error('logstack')).stack;
+            }
 
             for ( const logger of this.loggers ) {
                 logger.onLogMessage(

src/backend/src/modules/core/lib/log.js

@@ -38,7 +38,7 @@ const config = require('../../../config.js');
 *   @param {Object} objects - Objects to be logged.
 * @returns {string} A formatted string representation of the log entry.
 */
-const stringify_log_entry = ({ prefix, log_lvl, crumbs, message, fields, objects }) => {
+const stringify_log_entry = ({ prefix, log_lvl, crumbs, message, fields, objects, stack }) => {
     const { colorize } = require('json-colorizer');
 
     let lines = [], m;
@@ -94,6 +94,7 @@ const stringify_log_entry = ({ prefix, log_lvl, crumbs, message, fields, objects
         }
 
         if ( k === 'timestamp' ) continue;
+        if ( k === 'stack' ) continue;
         let v; try {
             v = colorize(JSON.stringify(fields[k]));
         } catch (e) {
@@ -102,6 +103,9 @@ const stringify_log_entry = ({ prefix, log_lvl, crumbs, message, fields, objects
         m += ` \x1B[1m${k}:\x1B[0m ${v}`;
         lf();
     }
+    if ( fields.stack ) {
+        lines.push(fields.stack);
+    }
     return lines.join('\n');
 };