influxdb database measurements is wrong from influxdb /debug/vars

List the minimal actions needed to reproduce the behavior.

1. curl influxdb url：influxdb:8086/debug/vars

* InfluxDB version: 1.8.3
* Other relevant environment details: kubernetes

Output:
I found the number of apm database measurements is -31，but actually is 29
```
"database:apm": {
"name": "database",
"tags": {
"database": "apm"
},
"values": {
"numMeasurements": -31,
"numSeries": 127
}
},
```
```
> use apm;
Using database apm
> show measurements
name: measurements
name
----
database_access_cpm
database_access_percentile
database_access_resp_time
database_access_sla
instance_jvm_cpu
instance_jvm_memory_heap
instance_jvm_memory_heap_max
instance_jvm_memory_noheap
instance_jvm_memory_noheap_max
instance_jvm_old_gc_count
instance_jvm_old_gc_time
instance_jvm_thread_daemon_count
instance_jvm_thread_live_count
instance_jvm_thread_peak_count
instance_jvm_young_gc_count
instance_jvm_young_gc_time
instance_traffic
service_instance_relation_client_call_sla
service_instance_relation_client_cpm
service_instance_relation_client_percentile
service_instance_relation_client_resp_time
service_instance_relation_client_side
service_relation_client_call_sla
service_relation_client_cpm
service_relation_client_percentile
service_relation_client_resp_time
service_relation_client_side
service_traffic
ui_template
```

Start by reproducing the issue with curl against the InfluxDB /debug/vars endpoint, then compare its database:apm numMeasurements value with the results of SHOW MEASUREMENTS. Done means the reported measurement count matches the actual measurements for the apm database.

Written by the indexing model from the issue text.