Fixing panic seen in TM when timestamp_ms appears as float and not string

[rimashah25]

Resolving the panic seen in a stack trace.
Aug 17 16:53:29 : panic: interface conversion: interface {} is float64, not string

---

Which Traffic Control components are affected by this PR?

• Traffic Monitor

What is the best way to verify this PR?

Run unit tests and when installing TM in an environment, ensure that it starts up without a panic.

If this is a bugfix, which Traffic Control versions contained the bug?

7.0.1

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Merging #7730 (411b048) into master (75ec56f) will decrease coverage by 35.75%.
Report is 70 commits behind head on master.
The diff coverage is 9.38%.

@@              Coverage Diff              @@
##             master    #7730       +/-   ##
=============================================
- Coverage     65.05%   29.30%   -35.75%     
  Complexity       98       98               
=============================================
  Files           314      593      +279     
  Lines         12365    74418    +62053     
  Branches        907       90      -817     
=============================================
+ Hits           8044    21808    +13764     
- Misses         3968    50531    +46563     
- Partials        353     2079     +1726     

Flag	Coverage Δ
golib_unit	52.62% <37.76%> (?)
grove_unit	12.02% <ø> (?)
t3c_unit	5.99% <3.31%> (?)
traffic_monitor_unit	26.33% <0.00%> (?)
traffic_ops_unit	22.43% <5.14%> (?)
traffic_portal_v2	?
traffic_stats_unit	10.76% <ø> (?)
unit_tests	26.04% <9.38%> (-47.74%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
cache-config/t3c-apply/config/config.go	0.00% <0.00%> (ø)
cache-config/t3c-apply/torequest/cmd.go	0.00% <0.00%> (ø)
cache-config/t3c-check-refs/t3c-check-refs.go	1.78% <0.00%> (ø)
cache-config/t3c-generate/cfgfile/varnish.go	0.00% <0.00%> (ø)
cache-config/t3c-generate/config/config.go	0.81% <0.00%> (-0.08%)	⬇️
cache-config/t3c-generate/t3c-generate.go	0.00% <ø> (ø)
lib/go-atscfg/parentabstraction.go	79.47% <ø> (ø)
lib/go-tc/cdns.go	0.00% <ø> (ø)
lib/go-tc/deliveryservice_servers.go	0.00% <0.00%> (ø)
lib/go-tc/federation_resolver.go	45.45% <0.00%> (ø)
... and 32 more

... and 457 files with indirect coverage changes

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