Refactor to default to structured logging.

- Added `flattenMessage` and `FlattenedLogMessage` to default to
  structured logging.
- Rework the logic formatting messages for the ConsoleLogger.
- Add a more comprehensive README.

src/log-service.ts

@@ -1,10 +1,16 @@
 import { LogLevel } from './log-message';
-import Logger from './logger';
+import { Logger } from './logger';
 
 const ROOT_LOGGER_NAME = 'ROOT';
 
+/**
+ * Service for managing loggers. A LogService instance defines
+ * the service context for a set of loggers. Typically there is only one
+ * LogService instance per application, the one exported as default from this
+ * module.
+ */
 export class LogService {
-  private loggers: { [key: string]: Logger };
+  private loggers: Record<string, Logger>;
 
   public get ROOT_LOGGER() {
     return this.loggers[ROOT_LOGGER_NAME];
@@ -15,10 +21,38 @@ export class LogService {
     this.loggers[ROOT_LOGGER_NAME] = new Logger(
       ROOT_LOGGER_NAME,
       undefined,
-      LogLevel.ALL
+      LogLevel.ALL,
     );
   }
 
+  /**
+   * Get a logger by name. If the logger does not exist, it will be created.
+   * Loggers are hierarchical, with the heirarchy defined by the logger name.
+   * When creating a new logger, the parent logger is determined by the longest
+   * existing logger name that is a prefix of the new logger name. For example,
+   * if a logger with the name "foo" already exists, any subsequent loggers
+   * with the names "foo.bar", "foo/bar", "foolish.choice", etc. will be
+   * created as children of the "foo" logger.
+   *
+   * As another example, given the following invocations:
+   *
+   * ```typescript
+   * logService.getLogger('foo');
+   * logService.getLogger('foo.bar');
+   * logService.getLogger('foo.bar.baz');
+   * logService.getLogger('foo.qux');
+   * ```
+   *
+   * will result in the following logging hierarchy:
+   *
+   *     foo
+   *     ├─foo.bar
+   *     │ └─foo.bar.baz
+   *     └─foo.qux
+   *
+   * See the Logger class for details on how loggers are used and the
+   * implications of the logger hierarchy.
+   */
   public getLogger(name: string, threshold?: LogLevel): Logger {
     if (this.loggers[name]) {
       return this.loggers[name];
@@ -30,7 +64,7 @@ export class LogService {
       .filter((n: string) => name.startsWith(n))
       .reduce(
         (acc: string, cur: string) => (acc.length > cur.length ? acc : cur),
-        ''
+        '',
       );
 
     if (parentLoggerName) {