…y (v1.31.x backport) (#7293) (#7301)

A user has reported a GOAWAY with too_many_pings when using BDP. We
aren't certain why it is happening, but want to provide a way to disable
BDP while we continue investigating. b/162162973

…headers (v1.31.x backport) (#7224) (#7313)

Expose "content-type" header (hard-coded) at header matching, pretend it's already there.

… interval measurement (v1.31.x backport) (#7209) (#7315)

- Add support for send_all_clusters field in LRS response. When it is set to true, just send load reports for clusters that the client is currently tracking (aka, is sending load to).

- The actual load report interval (in each ClusterStats message, which contains the stats for each cluster:eds_service) should be tracked individually.

…t) (#7299) (#7317)

Move the creation of LoadStatsStore (aka, the stats object) into XdsClient. The XdsClient is responsible for managing the lifetime of stats objects. Creations of LoadStatsStores are reference counted so that multiple EDS policies can retrieve the same stats object for load recording. Counters for recording loads per locality also need to be reference counted, as each EDS policy for the same cluster will receive endpoints for the same group of localities, they will use the same load counters for recording each locality's loads.

Followup to #7267. The rest of the README page refers to v1.31.0, so it would seem reasonable to link to the example folders of v1.31.0 too -- rather than v1.30.0.

@ejona86