prometheusremotewrite exporter crashes collector with metrics names < 6 characters

[dan-vectra]

Describe the bug
When Collector uses prometheusremotewriteexporter, metric names longer than 6 characters cause it to crash

2021-03-03 09:30:54
panic: runtime error: slice bounds out of range [-1:]

2021-03-03 09:30:54
goroutine 97 [running]:

2021-03-03 09:30:54
go.opentelemetry.io/collector/exporter/prometheusremotewriteexporter.getPromMetricName(0xc0003df7c0, 0x0, 0x0, 0x6, 0x6)

Seems to be caused by this logic which checks for "total" at the end of the metric name:

opentelemetry-collector/exporter/prometheusremotewriteexporter/helper.go

Line 197 in 68c5961

isCounter = isCounter && name[len(name)-len(totalStr):] != totalStr

Steps to reproduce
Enable Prometheus Remote Write exporter, send OTLP metrics to collector with names shorter than 6 characters

What version did you use?
Version: 0.16.0

What config did you use?
Config:
extensions:
health_check:
port: 13133

receivers:
otlp:
protocols:
grpc:
endpoint: 0.0.0.0:4317
http:

processors:
batch:

exporters:
logging:
logLevel: warn
prometheusremotewrite:
endpoint: "http://127.0.0.1:9201/write"

service:
pipelines:
traces:
receivers: [otlp]
processors: [batch]
exporters: [logging]

metrics:
  receivers: [otlp]
  processors: [batch]
  exporters: [logging, prometheusremotewrite]

extensions: [health_check]

Environment
OS: alpine:3.12 (Docker)