fix: prevent exporter panic on malformed metric lines

[haydaramru]

Summary

The handleMetric() function in prometheus/main.go splits incoming metric lines using strings.Fields() and accesses the resulting slice without bounds checks. If a line is malformed or too short (e.g. an empty MQTT topic producing fewer whitespace-delimited fields), this can trigger an index-out-of-range panic and crashing the entire Prometheus exporter, disabling telemetry for all tarpits.

This is the same underlying issue as #8, but the problem is systemic and not limited to SUBSCRIBE:

Case	Unguarded access	Panics if fields <
connect	fields[2]	3
disconnect	fields[3]	4
M-SEARCH	fields[2]	3
non-M-SEARCH	fields[2]	3
CONNECT	fields[2]	3
SUBSCRIBE	fields[2], fields[3]	4
PUBLISH	fields[2], fields[3]	4

The credentials and otherHttpRequests cases already have proper bounds checks and remain unchanged.

Fix: Introduce a top-level guard len(fields) < 2 before accessing fields[0]/fields[1] and per-case guards for higher indices. Malformed lines are now logged and skipped instead of crashing the process.

Resolves #8.