Issue Description

I have stellar-core 12.5.0-32 that exposes negative values of the overlay.delay.write-queue metric (for example sum or mean):

,"overlay.delay.write-queue":{
"type":"timer",
"count":12216602352,
"event_type":"calls",
"rate_unit":"s",
"mean_rate":4504.95,
"1_min_rate":6277.53,
"5_min_rate":6355.12,
"15_min_rate":6257.9,
"duration_unit":"ms",
"min":0,
"max":149469,
"mean":-633.807,
"stddev":7763.77,
"sum":-7.74296e+12,
"median":0,
"75%":0.001,
"95%":5.3996,
"98%":2962.81,
"99%":14027.3,
"99.9%":66916.1
}

Steps to Reproduce

On core 12.5.0-32: curl 127.0.0.1:11626/metrics and look at metrics

Expected Result

Timers have positive values

Actual Result

overlay.delay.write-queue has negative sum and mean. In my case this is the only affected metric.

Your Environment and Setup

• stellar-core 12.5.0-32
• Ubuntu 16.04.6 LTS
• installed using apt.stellar.org deb package

Additional context

I noticed this because the stellar-core-prometheus-exporter stops returning data when it sees negative values for sum.