Compactor: pyroscope_compaction_size_bytes histogram buckets are too small

## Description

The `pyroscope_compaction_size_bytes` histogram has bucket boundaries that are far too small to capture actual compacted block sizes, making the metric effectively useless.

## Current bucket definition

https://github.com/grafana/pyroscope/blob/main/pkg/compactor/bucket_compactor.go#L290-L294

```go
m.Size = prometheus.NewHistogramVec(prometheus.HistogramOpts{
Name: "pyroscope_compaction_size_bytes",
Help: "Final block size after compaction by level",
Buckets: prometheus.ExponentialBuckets(32, 1.5, 12),
}, []string{"level"})
```

This generates buckets: `32, 48, 72, 108, 162, 243, 364, 546, 820, 1230, 1845, 2768` — maxing out at **~2.7 KB**.

## Problem

Compacted blocks are typically in the **MB to GB** range. Every observation lands in the `+Inf` bucket, so `histogram_quantile()` returns a flat value near the last finite bucket boundary (~3KB) regardless of actual block sizes.

```promql
-- Always returns ~3KB regardless of actual compaction sizes
histogram_quantile(0.95, sum(rate(pyroscope_compaction_size_bytes_bucket{cluster="$cluster"}[15m])) by (le))
```

## Suggested fix

Use buckets that cover the realistic range of compacted block sizes (MB to GB):

```go
Buckets: prometheus.ExponentialBuckets(1<<20, 2, 15),
// 1MB, 2MB, 4MB, 8MB, 16MB, 32MB, 64MB, 128MB, 256MB, 512MB, 1GB, 2GB, 4GB, 8GB, 16GB
```

Alternatively, this could be made configurable so operators can tune the bucket boundaries to match their workload characteristics.