Enable storing service graph metrics as native histograms

[ppcano]

Relates to pull/293-OSS Docker setup: store span metrics as native histograms

The exponential_histogram_max_size option in the otelcol.connector.servicegraph collector added native histogram support, however, this option is not yet available in the Alloy config.

Alloy v12.0 supports the exponential_histogram_max_size option (see the internal Slack thread and grafana/alloy#4816).

Alloy was upgraded. We can now enable the exponential_histogram_max_size in the Alloy config.