Zero value wall times cause backend failures when spans are uploaded

[philwebb]

Describe the bug
Please see spring-projects/spring-boot#38579 for background.

Environment

• Micrometer version 1.12.0

To Reproduce
How to reproduce the bug:
See reproducer in linked issue.

Expected behavior
The JSON payload does not include invalid timestamps:

  {                                                                                                       
    "traceId": "XXXX",                                                        
    "parentId": "XXXX",                                                                       
    "id": "XXXX",                                                                             
    "kind": "CLIENT",                                                                                     
    "name": "connection",                                                                                 
    "timestamp": 1702045951598046,                                                                        
    "duration": 6111,                                                                                     
    "localEndpoint": {                                                                                    
      "serviceName": "XXXX",
      "ipv4": "XXXX"
    },                                                                                                    
    "annotations": [                                                                                      
      {                                                                                                   
        "timestamp": 0,                                                                                   
        "value": "\"acquired\":{}"                                                                        
      },                                                                                                  
      {                                                                                                   
        "timestamp": 0,                                                                                   
        "value": "\"commit\":{}"                                                                          
      }                                                                                                   
    ],                                                                                                    
    "tags": {                                                                                             
      "jdbc.datasource.driver": "org.postgresql.Driver",                                                  
      "jdbc.datasource.pool": "HikariPool-1",                                                             
      "otel.library.name": "org.springframework.boot",                                                    
      "otel.library.version": "3.2.0",                                                                    
      "otel.scope.name": "org.springframework.boot",                                                      
      "otel.scope.version": "3.2.0"                                                                       
    }                                                                                                     
  }, 

Additional context
In 1.12.0 the Observation.Event.getWallTime() method was added with a default return of 0. Users that call Event.of will have an event that includes the correct walltime, but users that implement Event directly will always return 0.

These 0 values are not valid and should not be passed to the backend.

[jonatan-ivanov]

Fixed by #487