Cut 2.20.0-rc.1

Note that all new lines in the CHANGELOG except the last will
disappear in the final release because they only deal with issues
introduced in 2.20.0-rc.0.

This commit also adds an explanation for the WAL compression becoming
default.

Furthermore, this commit reverts the upgrade to grpc 1.30.0. It was
just a routine upgrade, but because grpc 1.30.0 contains two very
Go-module unfriendly breaking changes that in combination wreak havoc
for anyone importing from the prometheus/prometheus repo while still
depending on 1.29.1, we decided to revert as Prometheus itself doesn't
need the upgrade right now.

Signed-off-by: beorn7 <beorn@grafana.com>

CHANGELOG.md

@@ -1,5 +1,14 @@
+## 2.20.0-rc.1 / 2020-07-20
+
+* [ENHANCEMENT] SD: Improve Docker Swarm example. #7608
+* [ENHANCEMENT] SD: Enable use of Unix socket for Docker Swarm. #7604
+* [BUGFIX] SD: Don't panic on an empty Docker Swarm config. #7615
+* [BUGFIX] TSDB: Don't panic on WAL corruptions. #7550
+
 ## 2.20.0-rc.0 / 2020-07-16
 
+This release changes WAL compression from opt-in to default. WAL compression will prevent a downgrade to v2.10 or earlier without deleting the WAL. Disable WAL compression explicitly by setting the command line flag `--no-storage.tsdb.wal-compression` if you require downgrading to v2.10 or earlier.
+
 * [CHANGE] promtool: Changed rule numbering from 0-based to 1-based when reporting rule errors. #7495
 * [CHANGE] Remote read: Added `prometheus_remote_storage_read_queries_total` counter and `prometheus_remote_storage_read_request_duration_seconds` histogram, removed `prometheus_remote_storage_remote_read_queries_total` counter.
 * [CHANGE] Remote write: Added buckets for longer durations to `prometheus_remote_storage_sent_batch_duration_seconds` histogram.

VERSION

@@ -1 +1 @@
-2.20.0-rc.0
+2.20.0-rc.1