feat: add metric and log to track dropped samples

[itatabitovski]

Introduce new metric graphite_dropped_samples_total to track any samples that are dropped either because of the explicit mapping config drop action or because of the --graphite.mapping-strict-match flag.

Also log a warning for each dropped metric.

It may make sense to split the dropping reasons and track them separately.

❯ echo 'xxxxxxx.xxxd 1234 123213' | nc -w1 127.0.0.1 9109

❯ ./graphite_exporter --graphite.mapping-strict-match
time=2025-04-12T10:58:18.816+02:00 level=INFO source=main.go:84 msg="Starting graphite_exporter" version_info="(version=0.16.0, branch=dropped-metrics, revision=04e6da143c01cbaede1f005107235da44ca465d6)"
time=2025-04-12T10:58:18.817+02:00 level=INFO source=main.go:85 msg="(go=go1.23.6, platform=linux/amd64, user=ilin@nuc, date=20250412-08:24:17, tags=unknown)"
time=2025-04-12T10:58:18.817+02:00 level=INFO source=tls_config.go:347 msg="Listening on" address=[::]:9108
time=2025-04-12T10:58:18.817+02:00 level=INFO source=tls_config.go:350 msg="TLS is disabled." http2=false address=[::]:9108
time=2025-04-12T10:58:27.907+02:00 level=WARN source=collector.go:159 msg="Dropped line" line="xxxxxxx.xxxd 1234 123213"

❯ curl -s localhost:9108/metrics | grep dropped_sa
# HELP graphite_dropped_samples_total Total count of samples dropped
# TYPE graphite_dropped_samples_total counter
graphite_dropped_samples_total 1

@matthiasr
@pedro-stanaka