Add optional observability

[samber]

metricTTLSecondDesc = prometheus.NewDesc(
	"cache_settings_ttl_second",
	"In-memory cache TTL duration.",
	[]string{"name"}, nil,
)

metricCapacityDesc = prometheus.NewDesc(
	"cache_settings_capacity",
	"In-memory cache capacity.",
	[]string{"name"}, nil,
)

metricMemByteDesc = prometheus.NewDesc(
	"cache_mem_byte",
	"In-memory cache RAM usage.",
	[]string{"name"}, nil,
)

metricLengthDesc = prometheus.NewDesc(
	"cache_length",
	"In-memory cache length.",
	[]string{"name"}, nil,
)

metricInsertionDesc = prometheus.NewDesc(
	"cache_insertion_total",
	"In-memory cache insertions.",
	[]string{"name"}, nil,
)

metricHitDesc = prometheus.NewDesc(
	"cache_hit_total",
	"In-memory cache hits.",
	[]string{"name"}, nil,
)

metricMissDesc = prometheus.NewDesc(
	"cache_miss_total",
	"In-memory cache misses.",
	[]string{"name"}, nil,
)

metricEvictionDesc = prometheus.NewDesc(
	"cache_eviction_total",
	"In-memory cache evictions.",
	[]string{"name"}, nil,
)