[Synthetics] Update parsing to skip replacing missing values (elastic…

…#192662)

## Summary

Based on user reported issue in
https://discuss.elastic.co/t/elastic-lightweight-monitor-host-name-no-longer-using-current-host-name/366421

Update parsing to skip replacing missing values !!

### Before

https://${host.name} would be replaced with "https://",


### After
Var value remain as is so that it would look for env variable 
"https://${host.name}", 


### Testing

can be tested in the ui , go to add monitor ui , user params in url
field and use inspect configuration to see parsed value, enabled inspect
button from kibana advanced settings observability inspect ES queries !!

<img width="852" alt="image"
src="https://github.com/user-attachments/assets/66bf460a-7d63-4fb3-993d-1caaa6812583">


<img width="1106" alt="image"
src="https://github.com/user-attachments/assets/6f5e97c6-182a-4cd1-a345-976c321d449a">

---------

Co-authored-by: Justin Kambic <jk@elastic.co>

x-pack/plugins/observability_solution/synthetics/server/synthetics_service/formatters/formatting_utils.test.ts

@@ -40,10 +40,10 @@ describe('replaceStringWithParams', () => {
     expect(result).toEqual('https://elastic.co');
   });
 
-  it('returns empty value in case no param', () => {
+  it('returns same value in case no param', () => {
     const result = replaceStringWithParams('${homePageUrl}', {}, logger);
 
-    expect(result).toEqual('');
+    expect(result).toEqual('${homePageUrl}');
   });
 
   it('works on objects', () => {
@@ -76,6 +76,46 @@ describe('replaceStringWithParams', () => {
     expect(result).toEqual('Basic https://elastic.co https://elastic.co/product');
   });
 
+  it('works on multiple without spaces', () => {
+    const result = replaceStringWithParams(
+      'Basic ${homePageUrl}${homePageUrl1}',
+      { homePageUrl: 'https://elastic.co', homePageUrl1: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('Basic https://elastic.cohttps://elastic.co/product');
+  });
+
+  it('works on multiple without spaces and one missing', () => {
+    const result = replaceStringWithParams(
+      'Basic ${homePageUrl}${homePageUrl1}',
+      { homePageUrl: 'https://elastic.co', homePageUrl2: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('Basic https://elastic.co${homePageUrl1}');
+  });
+
+  it('works on multiple without with default', () => {
+    const result = replaceStringWithParams(
+      'Basic ${homePageUrl}${homePageUrl1:test}',
+      { homePageUrl: 'https://elastic.co', homePageUrl2: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('Basic https://elastic.cotest');
+  });
+
+  it('works on multiple with multiple defaults', () => {
+    const result = replaceStringWithParams(
+      'Basic ${homePageUrl:test}${homePageUrl1:test4}',
+      { homePageUrl3: 'https://elastic.co', homePageUrl2: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('Basic testtest4');
+  });
+
   it('works with default value', () => {
     const result = replaceStringWithParams(
       'Basic ${homePageUrl:https://elastic.co} ${homePageUrl1}',
@@ -143,6 +183,36 @@ describe('replaceStringWithParams', () => {
     expect(result).toEqual('Basic  ${} value');
   });
 
+  it('works with ${host.name} for missing params', () => {
+    const result = replaceStringWithParams(
+      'Basic  ${host.name} value',
+      { homePageUrl1: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('Basic  ${host.name} value');
+  });
+
+  it('works with ${host.name} one missing params', () => {
+    const result = replaceStringWithParams(
+      'Basic ${host.name} ${homePageUrl1} value',
+      { homePageUrl1: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('Basic ${host.name} https://elastic.co/product value');
+  });
+
+  it('works with ${host.name} just missing params', () => {
+    const result = replaceStringWithParams(
+      '${host.name} ${homePageUrl1}',
+      { homePageUrl1: 'https://elastic.co/product' },
+      logger
+    );
+
+    expect(result).toEqual('${host.name} https://elastic.co/product');
+  });
+
   it('works with } ${abc} as part of value', () => {
     const result = replaceStringWithParams(
       'Basic } ${homePageUrl1} value',
@@ -162,4 +232,10 @@ describe('replaceStringWithParams', () => {
 
     expect(result).toEqual('Basic https://elastic.co/product { value');
   });
+
+  it("returns value as string | null when no params and it's an object", () => {
+    const result = replaceStringWithParams({}, { param: '1' }, logger);
+
+    expect(result).toEqual({});
+  });
 });

x-pack/plugins/observability_solution/synthetics/server/synthetics_service/formatters/formatting_utils.ts

@@ -6,6 +6,7 @@
  */
 
 import { Logger } from '@kbn/logging';
+import { isEmpty } from 'lodash';
 import { ConfigKey, MonitorFields } from '../../../common/runtime_types';
 import { ParsedVars, replaceVarsWithParams } from './lightweight_param_formatter';
 import variableParser from './variable_parser';
@@ -20,7 +21,7 @@ export const replaceStringWithParams = (
   params: Record<string, string>,
   logger?: Logger
 ) => {
-  if (!value || typeof value === 'boolean') {
+  if (!value || typeof value === 'boolean' || isEmpty(params)) {
     return value as string | null;
   }
 
@@ -42,6 +43,10 @@ export const replaceStringWithParams = (
 
     const parsedVars: ParsedVars = variableParser.parse(value);
 
+    if (allParamsAreMissing(parsedVars, params)) {
+      return value as string | null;
+    }
+
     return replaceVarsWithParams(parsedVars, params);
   } catch (e) {
     logger?.error(`error parsing vars for value ${JSON.stringify(value)}, ${e}`);
@@ -50,6 +55,19 @@ export const replaceStringWithParams = (
   return value as string | null;
 };
 
+const allParamsAreMissing = (parsedVars: ParsedVars, params: Record<string, string>) => {
+  const hasDefault = parsedVars.some(
+    (parsedVar) => parsedVar.type === 'var' && parsedVar.content.default
+  );
+  if (hasDefault) {
+    return false;
+  }
+  const varKeys = parsedVars
+    .filter((parsedVar) => parsedVar.type === 'var')
+    .map((v) => (typeof v.content === 'string' ? v.content : v.content.name));
+  return varKeys.every((v) => !params[v]);
+};
+
 const SHELL_PARAMS_REGEX = /\$\{[a-zA-Z_][a-zA-Z0-9\._\-?:]*\}/g;
 
 export const hasNoParams = (strVal: string) => {

x-pack/plugins/observability_solution/synthetics/server/synthetics_service/formatters/lightweight_param_formatter.test.ts

@@ -162,4 +162,11 @@ describe('LightweightParamFormatter', () => {
     const result = replaceVarsWithParams(formatter, params);
     expect(result).toEqual('https://default:1234');
   });
+  it('wraps content name when no default', () => {
+    const result = replaceVarsWithParams(
+      [{ type: 'var', content: { name: 'missing', default: null } }],
+      {}
+    );
+    expect(result).toEqual('${missing}');
+  });
 });

x-pack/plugins/observability_solution/synthetics/server/synthetics_service/formatters/lightweight_param_formatter.ts

@@ -16,7 +16,7 @@ export function replaceVarsWithParams(vars: ParsedVars, params: Record<string, s
       if (v.type === 'nonvar') {
         return v.content;
       }
-      return params[v.content.name]?.trim() || v.content.default;
+      return params[v.content.name]?.trim() || v.content.default || '${' + v.content.name + '}';
     })
     .join('');
 }