Rename db.client.connections.* metrics to db.client.connection.* and rename db.client.connection.usage to db.client.connection.count and adapting units

[gregkalapos]

Fixes #967
Fixes #201
Fixes #381

Changes

As described in #201 and discussed in the DB Semconv Stability WG meeting: db.client.connection.usage -> db.client.connection.count. Also fixing #967 and renaming the db.client.connections.* metrics namespace to db.client.connection.*.

Since this PR already changes Db metrics, also adapting unit from ms to s as described in #381.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[trask]

it may be worth fixing #967 in this PR also

[trask]

@gregkalapos sorry, I realized it would be good to add one more change to this PR: #381

bundling #381 here would have the advantage that we know the old name is always associated with milliseconds and the new name is always associated with seconds

(and it would affect the schema translation, where the metrics that changed from milliseconds to seconds would no longer be considered renames)

[gregkalapos]

CI fails, dotnet doesn't ❤️ me:

Run dotnet tool update -g docfx --version "3.0.0-*" --add-source https://docfx.pkgs.visualstudio.com/docfx/_packaging/docs-public-packages/nuget/v3/index.json
/tmp/837413a5-82a5-4ef9-abd4-39c1160faf4a/restore.csproj : error NU1102: Unable to find package docfx with version (>= 3.0.0)
/tmp/837413a5-82a5-4ef9-abd4-39c1160faf4a/restore.csproj : error NU1102:   - Found 224 version(s) in nuget.org [ Nearest version: 2.76.0 ]
/tmp/837413a5-82a5-4ef9-abd4-39c1160faf4a/restore.csproj : error NU1102:   - Found 0 version(s) in https://docfx.pkgs.visualstudio.com/docfx/_packaging/docs-public-packages/nuget/v3/index.json
Tool 'docfx' failed to update due to the following:
The tool package could not be restored.
Tool 'docfx' failed to install. This failure may have been caused by:

* You are attempting to install a preview release and did not use the --version option to specify the version.
* A package by this name was found, but it was not a .NET tool.
* The required NuGet feed cannot be accessed, perhaps because of an Internet connection problem.
* You mistyped the name of the tool.

For more reasons, including package naming enforcement, visit https://aka.ms/failure-installing-tool
Error: Process completed with exit code 1.

Update: issue about this: #987