Metrics cleanup

[fishy]

1. Change some of the references to statsd metrics in doc comments to prometheus metrics.
2. Remove the deprecated prometheus metrics/labels from the previous release.

While I'm here, also add go 1.20 to CI.

[kylelemons]

Can you update the PR description with a list of all of the changed / removed metrics so it's easy to copy/paste into release notes?

[fishy]

Removed metrics (all the replacements were already added from at least v0.9.12)

• httpbp_server_panic_recover_total: use httpbp_server_recovered_panics_total instead
• kafkabp_consumer_rebalance_total: use kafkabp_consumer_rebalances_total and kafkabp_consumer_rebalance_failures_total instead
• thriftbp_server_panic_recover_total: use thriftbp_server_recovered_panics_total instead
• thriftbp_client_pool_exhausted_total: use thriftbp_client_pool_exhaustions_total instead
• thriftbp_client_pool_release_error_total: use thriftbp_client_pool_release_errors_total instead
• thriftbp_client_pool_active_connections: use thrift_client_pool_active_connections instead
• thriftbp_client_pool_allocated_clients: use thrift_client_pool_idle_connections instead

Removed labels:

• thrift_client_name from thrift_client_pool_*: use thrift_pool instead.