Speed up build metric name.

[jeschkies]

› benchstat before.log after.log 
goos: linux
goarch: amd64
pkg: github.com/prometheus-community/yet-another-cloudwatch-exporter/pkg/promutil
cpu: AMD Ryzen 7 3700X 8-Core Processor             
                                                                      │  before.log  │              after.log              │
                                                                      │    sec/op    │   sec/op     vs base                │
_BuildMetricName/aws_elasticache_cpuutilization_average-16              1026.0n ± 1%   653.2n ± 1%  -36.34% (p=0.000 n=10)
_BuildMetricName/aws_sagemaker_trainingjobs_cpuutilization_average-16   1157.5n ± 2%   737.8n ± 2%  -36.26% (p=0.000 n=10)
_BuildMetricName/aws_glue_driver_aggregate_bytes_read_average-16        1040.5n ± 1%   624.2n ± 1%  -40.01% (p=0.000 n=10)
_BuildMetricName/aws_glue_aggregate_glue_jobs_bytes_read_average-16     1013.5n ± 1%   600.4n ± 2%  -40.76% (p=0.000 n=10)
geomean                                                                  1.058µ        651.9n       -38.38%