Average block time calculation neglects `startHeight`

[michaelfig]

Describe the bug
For a medium-to-large genesis.json initial_height (reflected in Meteor.settings.params.startHeight), average block time drops suddenly on the second block.

This is tested against big-dipper branch v0.41.x-14.2.

To Reproduce
Steps to reproduce the behavior:

1. Start bigdipper on a chain with settings.json params.startHeight: 64628, params.defaultBlockTime: 5000 and genesis.json initial_height: "64628"
2. Notice that "average block height" begins at 5000, but drops near 0 at the next block.
3. Look in mongodb:

> db.analytics.find()
{ "_id" : "NvHB3YkndzwgR5wcB", "height" : 64629, "precommits" : 119, "averageBlockTime" : 5000, "time" : "2021-07-01T19:00:01.766892137Z", "voting_power" : NaN }
{ "_id" : "4ghFqaYGjG7frrYTN", "height" : 64630, "precommits" : 119, "averageBlockTime" : 0.20611171282686058, "timeDiff" : 11555, "time" : "2021-07-01T19:00:13.321499387Z", "voting_power" : 6671 }
{ "_id" : "3dRft5qPxTiZA9u6Q", "height" : 64631, "precommits" : 119, "averageBlockTime" : 0.3008927604400365, "timeDiff" : 6126, "time" : "2021-07-01T19:00:19.447433509Z", "voting_power" : 6671 }
{ "_id" : "YSxxuQETMFsf22czQ", "height" : 64632, "precommits" : 119, "averageBlockTime" : 0.3948199034533977, "timeDiff" : 6071, "time" : "2021-07-01T19:00:25.518063569Z", "voting_power" : 6671 }
{ "_id" : "Pm8h4ZBYJAKXcN7Yh", "height" : 64633, "precommits" : 119, "averageBlockTime" : 0.4881407330620581, "timeDiff" : 6032, "time" : "2021-07-01T19:00:31.550446712Z", "voting_power" : 6671 }

It appears that the calculation (specifically, of the second row of analytics above), averageBlockTime does not take startHeight into account and is somehow using height directly.

Expected behavior

Average block time would hover around 5-10 seconds.

Screenshots

Desktop (please complete the following information):

• OS: MacOS
• Browser Brave
• Version 1.26.67

Smartphone (please complete the following information):
n/a

Additional context
Add any other context about the problem here.

[kwunyeung]

@michaelfig Thank you for reporting. I will look into this ASAP.