What are the four metric types in Prometheus?

Prometheus supports four core metric types:

1. Counter: Cumulative metric that only increases (or resets to zero). Used for requests served, errors occurred, etc.
2. Gauge: Metric that can go up and down. Used for temperature, memory usage, concurrent requests
3. Histogram: Samples observations and counts them in configurable buckets. Also provides sum and count
4. Summary: Similar to histogram but calculates configurable quantiles over a sliding time window

Example:

# Counter
http_requests_total{method="GET",status="200"} 1234

# Gauge  
memory_usage_bytes 8589934592

# Histogram
http_request_duration_seconds_bucket{le="0.1"} 100
http_request_duration_seconds_bucket{le="0.5"} 150