[GH-5] Expose agent's basic metrics

[hainenber]

The agent currently is a blackbox; there's no metrics that characterize how it is during operations. Therefore, we should expose or export a basic subset of metrics pertaining to agent's operations.

Ideally, the metrics should be exported in OTEL format to ensure cross-usage with different observability platforms.

What I can think of at the moment (not meant to be exhaustive)

• Count of ingested logs
• Count of forwarded logs
• Count of internal errors
• Count of received restart signals
• Count of initialized components (inputs, tailers, buffers and forwarders)