Update metrics doc to add instructions for scraping etcd metrics

[rajeshvenkata]

Issue #, if available:
#7106

Description of changes:

• Update the metrics doc to include instructions for scraping etcd metrics
• Update design doc and image to include etcd

Testing (if applicable):

$ curl -H "Authorization: Bearer ${TOKEN}" "http://${NODE_IP}:2381/metrics"
# HELP etcd_cluster_version Which version is running. 1 for 'cluster_version' label with current cluster version
# TYPE etcd_cluster_version gauge
etcd_cluster_version{cluster_version="3.5"} 1
# HELP etcd_debugging_auth_revision The current revision of auth store.
# TYPE etcd_debugging_auth_revision gauge
etcd_debugging_auth_revision 1
# HELP etcd_debugging_disk_backend_commit_rebalance_duration_seconds The latency distributions of commit.rebalance called by bboltdb backend.
# TYPE etcd_debugging_disk_backend_commit_rebalance_duration_seconds histogram
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.001"} 67367
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.002"} 67368
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.004"} 67369
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.008"} 67369
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.016"} 67369
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.032"} 67369
etcd_debugging_disk_backend_commit_rebalance_duration_seconds_bucket{le="0.064"} 67369

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[sp1999]

/cherry-pick release-0.21

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@sp1999: once the present PR merges, I will cherry-pick it on top of release-0.21 in a new PR and assign it to you.

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[sp1999]

/cherry-pick release-0.20

[eks-distro-pr-bot]

@sp1999: once the present PR merges, I will cherry-pick it on top of release-0.20 in a new PR and assign it to you.

In response to this:

/cherry-pick release-0.20

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[sp1999]

/cherry-pick release-0.19

[eks-distro-pr-bot]

@sp1999: once the present PR merges, I will cherry-pick it on top of release-0.19 in a new PR and assign it to you.

In response to this:

/cherry-pick release-0.19

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.38%. Comparing base (7e4cf0e) to head (b20d665).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9089      +/-   ##
==========================================
- Coverage   72.38%   72.38%   -0.01%     
==========================================
  Files         578      578              
  Lines       45469    45469              
==========================================
- Hits        32915    32912       -3     
- Misses      10824    10826       +2     
- Partials     1730     1731       +1     

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