[synthtrace] Add usage examples and available options (#172833)

## Summary

Minor DX improvements to `synthtrace` CLI:
- Added list of available scenarios
- Added list of available log levels
- Added usage examples

## Screenshot

<img width="993" alt="Screenshot 2023-12-07 at 14 46 31"
src="https://github.com/elastic/kibana/assets/190132/c0d3623f-0b0f-420d-9f6f-5d013494658f">

packages/kbn-apm-synthtrace/src/cli/run_synthtrace.ts

@@ -8,17 +8,25 @@
 import datemath from '@kbn/datemath';
 import { Argv } from 'yargs';
 import yargs from 'yargs/yargs';
+import { readdirSync } from 'fs';
+import path from 'path';
 import { intervalToMs } from './utils/interval_to_ms';
 import { parseRunCliFlags } from './utils/parse_run_cli_flags';
 import { startHistoricalDataUpload } from './utils/start_historical_data_upload';
 import { startLiveDataUpload } from './utils/start_live_data_upload';
 
+function getBuiltinScenarios() {
+  return readdirSync(path.resolve(__dirname, '../scenarios')).map((s) => s.replace(/\.ts$/, ''));
+}
+
 function options(y: Argv) {
   return y
+    .usage('$0 <file>')
     .positional('file', {
-      describe: 'File that contains the trace scenario',
+      describe: 'Name of scenario',
       demandOption: true,
       string: true,
+      choices: getBuiltinScenarios(),
     })
     .option('target', {
       describe: 'Elasticsearch target',
@@ -54,6 +62,7 @@ function options(y: Argv) {
     })
     .option('logLevel', {
       describe: 'Log level',
+      choices: ['trace', 'debug', 'info', 'error'],
       default: 'info',
     })
     .option('scenarioOpts', {
@@ -66,7 +75,14 @@ function options(y: Argv) {
       describe: 'Assumes passed package version to avoid calling Fleet API to install',
       string: true,
     })
-    .showHelpOnFail(false);
+    .example(
+      '$0 simple_logs --target=http://admin:changeme@localhost:9200',
+      'Ingest data to specific Elasticsearch cluster'
+    )
+    .example('$0 simple_logs --live', 'Continuously ingest data to local development cluster')
+    .example('$0 simple_logs --from=now-24h --to=now', 'Ingest data for a fixed time window')
+    .showHelpOnFail(false)
+    .wrap(null);
 }
 
 async function run(argv: RunCliFlags) {